Rollup merge of #143194 - folkertdev:fix-single-element-simd-bitcast, r=workingjubilee

fix bitcast of single-element SIMD vectors

in effect this reverts https://github.com/rust-lang/rust/pull/142768 and adds additional tests. That PR relaxed the conditions on an early return in an incorrect way that would create broken LLVM IR.

https://godbolt.org/z/PaaGWTv5a

```rust
#![feature(repr_simd)]

#[repr(simd)]
#[derive(Clone, Copy)]
struct S([i64; 1]);

#[no_mangle]
pub extern "C" fn single_element_simd(b: S) -> i64 {
    unsafe { std::mem::transmute(b) }
}
```
at the time of writing generates this LLVM IR, where the type of the return is different from the function's return type.

```llvm
define noundef i64 ``````@single_element_simd(<1`````` x i64> %b) unnamed_addr {
start:
  ret <1 x i64> %b
}
```

The test output is actually the same for the existing tests, showing that the change didn't actually matter for any tested behavior. It is probably a bit faster to do the early return, but, well, it's incorrect in general.

zullip thread: [#t-compiler > Is transmuting a &#96;T&#96; to &#96;Tx1&#96; (one-element SIMD vector) UB?](https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/Is.20transmuting.20a.20.60T.60.20to.20.60Tx1.60.20.28one-element.20SIMD.20vector.29.20UB.3F/with/526262799)
cc ``````@sayantn``````
r? ``````@scottmcm``````

compiler/rustc_codegen_ssa/src/mir/rvalue.rs

@@ -1117,7 +1117,7 @@ pub(super) fn transmute_immediate<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     // While optimizations will remove no-op transmutes, they might still be
     // there in debug or things that aren't no-op in MIR because they change
     // the Rust type but not the underlying layout/niche.
-    if from_scalar == to_scalar {
+    if from_scalar == to_scalar && from_backend_ty == to_backend_ty {
         return imm;
     }
 
@@ -1136,13 +1136,7 @@ pub(super) fn transmute_immediate<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     assume_scalar_range(bx, imm, from_scalar, from_backend_ty);
 
     imm = match (from_scalar.primitive(), to_scalar.primitive()) {
-        (Int(..) | Float(_), Int(..) | Float(_)) => {
-            if from_backend_ty == to_backend_ty {
-                imm
-            } else {
-                bx.bitcast(imm, to_backend_ty)
-            }
-        }
+        (Int(..) | Float(_), Int(..) | Float(_)) => bx.bitcast(imm, to_backend_ty),
         (Pointer(..), Pointer(..)) => bx.pointercast(imm, to_backend_ty),
         (Int(..), Pointer(..)) => bx.ptradd(bx.const_null(bx.type_ptr()), imm),
         (Pointer(..), Int(..)) => {