Reformat mir! macro invocations to use braces.

The `mir!` macro has multiple parts:
- An optional return type annotation.
- A sequence of zero or more local declarations.
- A mandatory starting anonymous basic block, which is brace-delimited.
- A sequence of zero of more additional named basic blocks.

Some `mir!` invocations use braces with a "block" style, like so:
```
mir! {
    let _unit: ();
    {
	let non_copy = S(42);
	let ptr = std::ptr::addr_of_mut!(non_copy);
	// Inside `callee`, the first argument and `*ptr` are basically
	// aliasing places!
	Call(_unit = callee(Move(*ptr), ptr), ReturnTo(after_call), UnwindContinue())
    }
    after_call = {
	Return()
    }
}
```
Some invocations use parens with a "block" style, like so:
```
mir!(
    let x: [i32; 2];
    let one: i32;
    {
	x = [42, 43];
	one = 1;
	x = [one, 2];
	RET = Move(x);
	Return()
    }
)
```
And some invocations uses parens with a "tighter" style, like so:
```
mir!({
    SetDiscriminant(*b, 0);
    Return()
})
```
This last style is generally used for cases where just the mandatory
starting basic block is present. Its braces are placed next to the
parens.

This commit changes all `mir!` invocations to use braces with a "block"
style. Why?

- Consistency is good.

- The contents of the invocation is a block of code, so it's odd to use
  parens. They are more normally used for function-like macros.

- Most importantly, the next commit will enable rustfmt for
  `tests/mir-opt/`. rustfmt is more aggressive about formatting macros
  that use parens than macros that use braces. Without this commit's
  changes, rustfmt would break a couple of `mir!` macro invocations that
  use braces within `tests/mir-opt` by inserting an extraneous comma.
  E.g.:
  ```
  mir!(type RET = (i32, bool);, { // extraneous comma after ';'
      RET.0 = 1;
      RET.1 = true;
      Return()
  })
  ```
  Switching those `mir!` invocations to use braces avoids that problem,
  resulting in this, which is nicer to read as well as being valid
  syntax:
  ```
  mir! {
      type RET = (i32, bool);
      {
	  RET.0 = 1;
	  RET.1 = true;
	  Return()
      }
  }
  ```

src/tools/miri/tests/fail/intrinsics/ptr_metadata_uninit_slice_data.rs

@@ -7,10 +7,12 @@ use std::intrinsics::mir::*;
 
 #[custom_mir(dialect = "runtime")]
 pub unsafe fn deref_meta(p: *const *const [i32]) -> usize {
-    mir!({
-        RET = PtrMetadata(*p); //~ ERROR: Undefined Behavior: using uninitialized data
-        Return()
-    })
+    mir! {
+        {
+            RET = PtrMetadata(*p); //~ ERROR: Undefined Behavior: using uninitialized data
+            Return()
+        }
+    }
 }
 
 fn main() {

src/tools/miri/tests/fail/intrinsics/ptr_metadata_uninit_slice_data.stderr

@@ -1,8 +1,8 @@
 error: Undefined Behavior: using uninitialized data, but this operation requires initialized memory
   --> $DIR/ptr_metadata_uninit_slice_data.rs:LL:CC
    |
-LL |         RET = PtrMetadata(*p);
-   |         ^^^^^^^^^^^^^^^^^^^^^ using uninitialized data, but this operation requires initialized memory
+LL |             RET = PtrMetadata(*p);
+   |             ^^^^^^^^^^^^^^^^^^^^^ using uninitialized data, but this operation requires initialized memory
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information

src/tools/miri/tests/fail/intrinsics/ptr_metadata_uninit_slice_len.rs

@@ -7,10 +7,12 @@ use std::intrinsics::mir::*;
 
 #[custom_mir(dialect = "runtime")]
 pub unsafe fn deref_meta(p: *const *const [i32]) -> usize {
-    mir!({
-        RET = PtrMetadata(*p); //~ ERROR: Undefined Behavior: using uninitialized data
-        Return()
-    })
+    mir! {
+        {
+            RET = PtrMetadata(*p); //~ ERROR: Undefined Behavior: using uninitialized data
+            Return()
+        }
+    }
 }
 
 fn main() {

src/tools/miri/tests/fail/intrinsics/ptr_metadata_uninit_slice_len.stderr

@@ -16,8 +16,8 @@ LL |         (*p.as_mut_ptr().cast::<[*const i32; 2]>())[0] = 4 as *const i32;
 error: Undefined Behavior: using uninitialized data, but this operation requires initialized memory
   --> $DIR/ptr_metadata_uninit_slice_len.rs:LL:CC
    |
-LL |         RET = PtrMetadata(*p);
-   |         ^^^^^^^^^^^^^^^^^^^^^ using uninitialized data, but this operation requires initialized memory
+LL |             RET = PtrMetadata(*p);
+   |             ^^^^^^^^^^^^^^^^^^^^^ using uninitialized data, but this operation requires initialized memory
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information

src/tools/miri/tests/fail/intrinsics/ptr_metadata_uninit_thin.rs

@@ -7,10 +7,12 @@ use std::intrinsics::mir::*;
 
 #[custom_mir(dialect = "runtime")]
 pub unsafe fn deref_meta(p: *const *const i32) -> () {
-    mir!({
-        RET = PtrMetadata(*p); //~ ERROR: Undefined Behavior: using uninitialized data
-        Return()
-    })
+    mir! {
+        {
+            RET = PtrMetadata(*p); //~ ERROR: Undefined Behavior: using uninitialized data
+            Return()
+        }
+    }
 }
 
 fn main() {

src/tools/miri/tests/fail/intrinsics/ptr_metadata_uninit_thin.stderr

@@ -1,8 +1,8 @@
 error: Undefined Behavior: using uninitialized data, but this operation requires initialized memory
   --> $DIR/ptr_metadata_uninit_thin.rs:LL:CC
    |
-LL |         RET = PtrMetadata(*p);
-   |         ^^^^^^^^^^^^^^^^^^^^^ using uninitialized data, but this operation requires initialized memory
+LL |             RET = PtrMetadata(*p);
+   |             ^^^^^^^^^^^^^^^^^^^^^ using uninitialized data, but this operation requires initialized memory
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information