Add a mir-opt GVN test for #128299

Co-authored-by: DianQK <dianqk@dianqk.net>

tests/mir-opt/simplify_aggregate_to_copy_miscompile.foo.GVN.diff

@@ -0,0 +1,72 @@
+- // MIR for `foo` before GVN
++ // MIR for `foo` after GVN
+
+  fn foo(_1: &mut Option<i32>) -> Option<i32> {
+      debug v => _1;
+      let mut _0: std::option::Option<i32>;
+      let mut _2: &std::option::Option<i32>;
+      let mut _3: &std::option::Option<i32>;
+      let _4: &&mut std::option::Option<i32>;
+      let mut _5: isize;
+      let mut _7: !;
+      let mut _8: std::option::Option<i32>;
+      let mut _9: i32;
+      let mut _10: !;
+      let mut _11: &mut std::option::Option<i32>;
+      scope 1 {
+          debug col => _6;
+          let _6: i32;
+      }
+
+      bb0: {
+-         StorageLive(_2);
++         nop;
+          StorageLive(_3);
+          StorageLive(_4);
+          _4 = &_1;
+-         _11 = deref_copy (*_4);
+-         _3 = &(*_11);
++         _11 = copy _1;
++         _3 = &(*_1);
+          _2 = get(move _3) -> [return: bb1, unwind unreachable];
+      }
+
+      bb1: {
+          StorageDead(_3);
+          _5 = discriminant((*_2));
+          switchInt(move _5) -> [1: bb2, otherwise: bb3];
+      }
+
+      bb2: {
+-         StorageLive(_6);
++         nop;
+          _6 = copy (((*_2) as Some).0: i32);
+          StorageLive(_8);
+-         _8 = Option::<i32>::None;
+-         (*_1) = move _8;
++         _8 = const Option::<i32>::None;
++         (*_1) = const Option::<i32>::None;
+          StorageDead(_8);
+          StorageLive(_9);
+          _9 = copy _6;
+-         _0 = Option::<i32>::Some(move _9);
++         _0 = copy (*_2);
+          StorageDead(_9);
+-         StorageDead(_6);
++         nop;
+          StorageDead(_4);
+-         StorageDead(_2);
++         nop;
+          return;
+      }
+
+      bb3: {
+          StorageLive(_10);
+          unreachable;
+      }
++ }
++ 
++ ALLOC0 (size: 8, align: 4) {
++     00 00 00 00 __ __ __ __                         │ ....░░░░
+  }
+

tests/mir-opt/simplify_aggregate_to_copy_miscompile.rs

@@ -0,0 +1,32 @@
+//! The `simplify_aggregate_to_copy` mir-opt introduced in
+//! <https://github.com/rust-lang/rust/pull/128299> caused a miscompile because the initial
+//! implementation
+//!
+//! > introduce[d] new dereferences without checking for aliasing
+//!
+//! This test demonstrates the behavior, and should be adjusted or removed when fixing and relanding
+//! the mir-opt.
+#![crate_type = "lib"]
+// skip-filecheck
+//@ compile-flags: -O -Zunsound-mir-opts
+//@ test-mir-pass: GVN
+#![allow(internal_features)]
+#![feature(rustc_attrs, core_intrinsics)]
+
+// EMIT_MIR simplify_aggregate_to_copy_miscompile.foo.GVN.diff
+#[no_mangle]
+fn foo(v: &mut Option<i32>) -> Option<i32> {
+    if let &Some(col) = get(&v) {
+        *v = None;
+        return Some(col);
+    } else {
+        unsafe { std::intrinsics::unreachable() }
+    }
+}
+
+#[no_mangle]
+#[inline(never)]
+#[rustc_nounwind]
+fn get(v: &Option<i32>) -> &Option<i32> {
+    v
+}