add regression test for #74739 (mir const-prop bug)

src/test/ui/issues/issue-74739.rs

@@ -0,0 +1,14 @@
+// compile-flags: -O
+// run-pass
+
+struct Foo {
+    x: i32,
+}
+
+pub fn main() {
+    let mut foo = Foo { x: 42 };
+    let x = &mut foo.x;
+    *x = 13;
+    let y = foo;
+    assert_eq!(y.x, 13); // used to print 42 due to mir-opt bug
+}