add test for const type_id misoptimization

Author: Lukas Markeffsky

tests/mir-opt/gvn_type_id_polymorphic.cursed_is_i32.GVN.diff

@@ -0,0 +1,13 @@
+- // MIR for `cursed_is_i32` before GVN
++ // MIR for `cursed_is_i32` after GVN
+  
+  fn cursed_is_i32() -> bool {
+      let mut _0: bool;
+  
+      bb0: {
+-         _0 = Eq(const cursed_is_i32::<T>::{constant#0}, const cursed_is_i32::<T>::{constant#1});
++         _0 = const false;
+          return;
+      }
+  }
+  

tests/mir-opt/gvn_type_id_polymorphic.rs

@@ -0,0 +1,22 @@
+//@ test-mir-pass: GVN
+//@ compile-flags: -C opt-level=2
+
+#![feature(core_intrinsics)]
+
+fn generic<T>() {}
+
+const fn type_id_of_val<T: 'static>(_: &T) -> u128 {
+    std::intrinsics::type_id::<T>()
+}
+
+// EMIT_MIR gvn_type_id_polymorphic.cursed_is_i32.GVN.diff
+fn cursed_is_i32<T: 'static>() -> bool {
+    // CHECK-LABEL: fn cursed_is_i32(
+    // CHECK: _0 = const false;
+    // CHECK-NEXT: return;
+    (const { type_id_of_val(&generic::<T>) } == const { type_id_of_val(&generic::<i32>) })
+}
+
+fn main() {
+    dbg!(cursed_is_i32::<i32>());
+}