Add regression test for SimplifyBranchSame miscompilation

src/test/ui/mir/simplify-branch-same.rs

@@ -0,0 +1,21 @@
+// Regression test for SimplifyBranchSame miscompilation.
+// run-pass
+
+macro_rules! m {
+    ($a:expr, $b:expr, $c:block) => {
+        match $a {
+            Lto::Fat | Lto::Thin => { $b; (); $c }
+            Lto::No => { $b; () }
+        }
+    }
+}
+
+pub enum Lto { No, Thin, Fat }
+
+fn f(mut cookie: u32, lto: Lto) -> u32 {
+    let mut _a = false;
+    m!(lto, _a = true, {cookie = 0});
+    cookie
+}
+
+fn main() { assert_eq!(f(42, Lto::Thin), 0) }