Auto merge of #85828 - scottmcm:raw-eq, r=oli-obk

Stop generating `alloca`s & `memcmp` for simple short array equality

Example:
```rust
pub fn demo(x: [u16; 6], y: [u16; 6]) -> bool { x == y }
```

Before:
```llvm
define zeroext i1 `@_ZN10playground4demo17h48537f7eac23948fE(i96` %0, i96 %1) unnamed_addr #0 {
start:
  %y = alloca [6 x i16], align 8
  %x = alloca [6 x i16], align 8
  %.0..sroa_cast = bitcast [6 x i16]* %x to i96*
  store i96 %0, i96* %.0..sroa_cast, align 8
  %.0..sroa_cast3 = bitcast [6 x i16]* %y to i96*
  store i96 %1, i96* %.0..sroa_cast3, align 8
  %_11.i.i.i = bitcast [6 x i16]* %x to i8*
  %_14.i.i.i = bitcast [6 x i16]* %y to i8*
  %bcmp.i.i.i = call i32 `@bcmp(i8*` nonnull dereferenceable(12) %_11.i.i.i, i8* nonnull dereferenceable(12) %_14.i.i.i, i64 12) #2, !alias.scope !2
  %2 = icmp eq i32 %bcmp.i.i.i, 0
  ret i1 %2
}
```
```x86
playground::demo: # `@playground::demo`
	sub	rsp, 32
	mov	qword ptr [rsp], rdi
	mov	dword ptr [rsp + 8], esi
	mov	qword ptr [rsp + 16], rdx
	mov	dword ptr [rsp + 24], ecx
	xor	rdi, rdx
	xor	esi, ecx
	or	rsi, rdi
	sete	al
	add	rsp, 32
	ret
```

After:
```llvm
define zeroext i1 `@_ZN4mini4demo17h7a8994aaa314c981E(i96` %0, i96 %1) unnamed_addr #0 {
start:
  %2 = icmp eq i96 %0, %1
  ret i1 %2
}
```
```x86
_ZN4mini4demo17h7a8994aaa314c981E:
	xor	rcx, r8
	xor	edx, r9d
	or	rdx, rcx
	sete	al
	ret
```

src/test/ui/intrinsics/intrinsic-raw_eq-const-padding.rs

@@ -0,0 +1,11 @@
+#![feature(core_intrinsics)]
+#![feature(const_intrinsic_raw_eq)]
+#![deny(const_err)]
+
+const BAD_RAW_EQ_CALL: bool = unsafe {
+    std::intrinsics::raw_eq(&(1_u8, 2_u16), &(1_u8, 2_u16))
+//~^ ERROR evaluation of constant value failed
+};
+
+pub fn main() {
+}

src/test/ui/intrinsics/intrinsic-raw_eq-const-padding.stderr

@@ -0,0 +1,9 @@
+error[E0080]: evaluation of constant value failed
+  --> $DIR/intrinsic-raw_eq-const-padding.rs:6:5
+   |
+LL |     std::intrinsics::raw_eq(&(1_u8, 2_u16), &(1_u8, 2_u16))
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ reading 4 bytes of memory starting at alloc2, but 1 byte is uninitialized starting at alloc2+0x1, and this operation requires initialized memory
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0080`.

src/test/ui/intrinsics/intrinsic-raw_eq-const.rs

@@ -0,0 +1,27 @@
+// run-pass
+
+#![feature(core_intrinsics)]
+#![feature(const_intrinsic_raw_eq)]
+#![deny(const_err)]
+
+pub fn main() {
+    use std::intrinsics::raw_eq;
+
+    const RAW_EQ_I32_TRUE: bool = unsafe { raw_eq(&42_i32, &42) };
+    assert!(RAW_EQ_I32_TRUE);
+
+    const RAW_EQ_I32_FALSE: bool = unsafe { raw_eq(&4_i32, &2) };
+    assert!(!RAW_EQ_I32_FALSE);
+
+    const RAW_EQ_CHAR_TRUE: bool = unsafe { raw_eq(&'a', &'a') };
+    assert!(RAW_EQ_CHAR_TRUE);
+
+    const RAW_EQ_CHAR_FALSE: bool = unsafe { raw_eq(&'a', &'A') };
+    assert!(!RAW_EQ_CHAR_FALSE);
+
+    const RAW_EQ_ARRAY_TRUE: bool = unsafe { raw_eq(&[13_u8, 42], &[13, 42]) };
+    assert!(RAW_EQ_ARRAY_TRUE);
+
+    const RAW_EQ_ARRAY_FALSE: bool = unsafe { raw_eq(&[13_u8, 42], &[42, 13]) };
+    assert!(!RAW_EQ_ARRAY_FALSE);
+}