Add regression test for matching on u8

src/test/mir-opt/matches_u8.rs

@@ -0,0 +1,21 @@
+// EMIT_MIR_FOR_EACH_BIT_WIDTH
+// EMIT_MIR matches_u8.exhaustive_match.MatchBranchSimplification.diff
+
+pub enum E {
+    A,
+    B,
+}
+
+// This only breaks on u8's, but probably still have to test i8.
+#[no_mangle]
+pub fn exhaustive_match(e: E) -> u8 {
+    match e {
+        E::A => 0,
+        E::B => 1,
+    }
+}
+
+fn main() {
+  assert_eq!(exhaustive_match(E::A), 0);
+  assert_eq!(exhaustive_match(E::B), 1);
+}