Mark simplify_aggregate_to_copy mir-opt as unsound

Mark the `simplify_aggregate_to_copy` mir-opt added in
<#128299>.

This **partially** reverts commit
e7386b3,
reversing changes made to 02b1be1. The
implementation is just marked as unsound, but the test changes are
reverted.

compiler/rustc_mir_transform/src/gvn.rs

@@ -1079,7 +1079,9 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
             }
         }
 
-        if let AggregateTy::Def(_, _) = ty
+        // unsound: https://github.com/rust-lang/rust/issues/132353
+        if tcx.sess.opts.unstable_opts.unsound_mir_opts
+            && let AggregateTy::Def(_, _) = ty
             && let Some(value) =
                 self.simplify_aggregate_to_copy(rvalue, location, &fields, variant_index)
         {

tests/codegen/enum/unreachable_enum_default_branch.rs

@@ -28,13 +28,11 @@ pub fn implicit_match(x: Int) -> bool {
 // The code is from https://github.com/rust-lang/rust/issues/110097.
 // We expect it to generate the same optimized code as a full match.
 // CHECK-LABEL: @if_let(
-// CHECK: start:
-// CHECK-NOT: zext
-// CHECK: select
+// CHECK-NEXT:  start:
 // CHECK-NEXT: insertvalue
 // CHECK-NEXT: insertvalue
 // CHECK-NEXT: ret
 #[no_mangle]
 pub fn if_let(val: Result<i32, ()>) -> Result<i32, ()> {
-    if let Ok(x) = val { Ok(x * 2) } else { Err(()) }
+    if let Ok(x) = val { Ok(x) } else { Err(()) }
 }