[Custom Descriptors] Fix exact types in RemoveUnusedBrs handling of SuccessOnlyIfNonNull (#7615)

In that case we dropped exactness, leading to an error on the new
testcase after optimization. Fix is to use

  .with(Nullable)

Author: kripken

src/passes/RemoveUnusedBrs.cpp

@@ -1008,8 +1008,13 @@ struct RemoveUnusedBrs : public WalkerPass<PostWalker<RemoveUnusedBrs>> {
             //       (...)
             //     (ref.null bot<X>)
             //   )
-            curr->ref = maybeCast(
-              curr->ref, Type(curr->getSentType().getHeapType(), Nullable));
+            //
+            // A RefCast is added in some cases, but this is still generally
+            // worth doing as the BrOnNonNull and the appended null may end up
+            // optimized with surrounding code.
+            auto* casted =
+              maybeCast(curr->ref, curr->getSentType().with(Nullable));
+            curr->ref = casted;
             curr->op = BrOnNonNull;
             curr->castType = Type::none;
             curr->type = Type::none;

test/lit/passes/remove-unused-brs-exact-only.wast

@@ -0,0 +1,38 @@
+;; NOTE: Assertions have been generated by update_lit_checks.py and should not be edited.
+
+;; RUN: wasm-opt %s -all  --remove-unused-brs -S -o - | filecheck %s
+
+;; Like "remove-unused-brs-exact.wast", but with exact types in the input so we
+;; cannot have a NO_CD mode.
+
+(module
+  ;; CHECK:      (type $foo (struct))
+  (type $foo (struct))
+
+  ;; CHECK:      (func $br_on_cast_to_non_null (type $1) (param $foo (ref null $foo)) (result (ref (exact $foo)))
+  ;; CHECK-NEXT:  (block $block (result (ref (exact $foo)))
+  ;; CHECK-NEXT:   (drop
+  ;; CHECK-NEXT:    (block (result nullref)
+  ;; CHECK-NEXT:     (br_on_non_null $block
+  ;; CHECK-NEXT:      (ref.cast (ref null (exact $foo))
+  ;; CHECK-NEXT:       (local.get $foo)
+  ;; CHECK-NEXT:      )
+  ;; CHECK-NEXT:     )
+  ;; CHECK-NEXT:     (ref.null none)
+  ;; CHECK-NEXT:    )
+  ;; CHECK-NEXT:   )
+  ;; CHECK-NEXT:   (unreachable)
+  ;; CHECK-NEXT:  )
+  ;; CHECK-NEXT: )
+  (func $br_on_cast_to_non_null (param $foo (ref null $foo)) (result (ref (exact $foo)))
+    ;; We can simplify the br_on_cast to a br_on_non_null plus a cast.
+    (block $block (result (ref (exact $foo)))
+     (drop
+        (br_on_cast $block (ref null $foo) (ref (exact $foo))
+          (local.get $foo)
+        )
+      )
+      (unreachable)
+    )
+  )
+)