[Custom Descriptors] Fix GTO after #7937

Do not leave unreachable ref.cast_desc instructions with descriptors
arguments that have been optimized to no longer be descriptors. This was
intended to be inluded in #7937.

Author: tlively

src/passes/GlobalTypeOptimization.cpp

@@ -761,6 +761,18 @@ struct GlobalTypeOptimization : public Pass {
         curr->index = newIndex;
       }
 
+      void visitRefCast(RefCast* curr) {
+        // Unreachable ref.cast_desc instructions would not have been counted as
+        // reading the descriptor field, so their descriptor operands may no
+        // longer be descriptors. This is invalid, so replace such casts
+        // entirely.
+        if (curr->type == Type::unreachable && curr->desc &&
+            curr->desc->type != Type::unreachable) {
+          assert(curr->ref->type == Type::unreachable);
+          replaceCurrent(curr->ref);
+        }
+      }
+
       void visitFunction(Function* curr) {
         if (needEHFixups) {
           EHUtils::handleBlockNestedPops(curr, *getModule());

test/lit/passes/gto-desc.wast

@@ -1396,3 +1396,34 @@
   )
 )
 
+;; We can optimize away the descriptor since its only use is unreachable, but
+;; we must update that use to remain valid despite being unreachable.
+(module
+  (rec
+    ;; CHECK:      (rec
+    ;; CHECK-NEXT:  (type $struct (sub (struct)))
+    (type $struct (sub (descriptor $desc (struct))))
+    ;; CHECK:       (type $desc (sub (struct)))
+    (type $desc (sub (describes $struct (struct))))
+  )
+  ;; CHECK:       (type $2 (func))
+
+  ;; CHECK:      (func $test (type $2)
+  ;; CHECK-NEXT:  (local $struct (ref $struct))
+  ;; CHECK-NEXT:  (local $desc (ref $desc))
+  ;; CHECK-NEXT:  (drop
+  ;; CHECK-NEXT:   (unreachable)
+  ;; CHECK-NEXT:  )
+  ;; CHECK-NEXT: )
+  (func $test
+    (local $struct (ref $struct))
+    (local $desc (ref $desc))
+    (drop
+      (ref.cast_desc (ref $struct)
+        (unreachable)
+        (struct.new $desc)
+      )
+    )
+  )
+)
+