No longer check aligment and non-NULLness on &

This breaks creating unaligned raw pointers via `&packed.field as *const _`, which needs to be legal.
Also it doesn't seem like LLVM still relies on this, see
* https://github.com/solson/miri/issues/244#issuecomment-315563640
* https://internals.rust-lang.org/t/rules-for-alignment-and-non-nullness-of-references/5430/16

We probably want to handle this invariant like the others that validation is concerned with, and only
check it on function boundaries for now.

src/eval_context.rs

@@ -682,10 +682,6 @@ impl<'a, 'tcx> EvalContext<'a, 'tcx> {
                         bug!("attempted to take a reference to an enum downcast lvalue"),
                 };
 
-                // Check alignment and non-NULLness.
-                let (_, align) = self.size_and_align_of_dst(ty, val)?;
-                self.memory.check_align(ptr, align)?;
-
                 self.write_value(val, dest, dest_ty)?;
             }
 

tests/compile-fail/int_ptr_cast.rs

@@ -1,5 +0,0 @@
-fn main() {
-    let x = 2usize as *const u32;
-    // This must fail because alignment is violated
-    let _ = unsafe { &*x }; //~ ERROR: tried to access memory with alignment 2, but alignment 4 is required
-}

tests/compile-fail/int_ptr_cast2.rs

@@ -1,5 +0,0 @@
-fn main() {
-    let x = 0usize as *const u32;
-    // This must fail because the pointer is NULL
-    let _ = unsafe { &*x }; //~ ERROR: invalid use of NULL pointer
-}

tests/compile-fail/reference_to_packed.rs

@@ -11,6 +11,6 @@ fn main() {
         x: 42,
         y: 99,
     };
-    let p = &foo.x; //~ ERROR tried to access memory with alignment 1, but alignment 4 is required
-    let i = *p;
+    let p = &foo.x;
+    let i = *p; //~ ERROR tried to access memory with alignment 1, but alignment 4 is required
 }

tests/compile-fail/unaligned_ptr_cast.rs

@@ -2,5 +2,5 @@ fn main() {
     let x = &2u16;
     let x = x as *const _ as *const u32;
     // This must fail because alignment is violated
-    let _ = unsafe { &*x }; //~ ERROR: tried to access memory with alignment 2, but alignment 4 is required
+    let _x = unsafe { *x }; //~ ERROR: tried to access memory with alignment 2, but alignment 4 is required
 }