Fix unsound optimization with explicit variant discriminants

src/test/ui/mir/issue-89485.rs

@@ -0,0 +1,18 @@
+// Regression test for issue #89485.
+
+// run-pass
+
+#[derive(Debug, Eq, PartialEq)]
+pub enum Type {
+    A = 1,
+    B = 2,
+}
+pub fn encode(v: Type) -> Type {
+    match v {
+        Type::A => Type::B,
+        _ => v,
+    }
+}
+fn main() {
+  assert_eq!(Type::B, encode(Type::A));
+}