[Custom Descriptors] Fix shared GTO placeholders (#7907)

When we optimize a descriptor to describe a placeholder type because the
original described type no longer needs a descriptor, we were not
previously ensuring that the sharedness of the placeholder matched the
sharedness of the descriptor type. Make sure the shareness matches to
avoid validation errors when rebuilding the types.

Author: tlively

src/passes/GlobalTypeOptimization.cpp

@@ -578,7 +578,9 @@ struct GlobalTypeOptimization : public Pass {
         // Until we've created all the placeholders, create a placeholder
         // describee type for the next descriptor that needs one.
         if (placeholderIt != parent.descriptorsOfPlaceholders.end()) {
-          typeBuilder[i].descriptor(getTempHeapType(placeholderIt->first));
+          auto descriptor = placeholderIt->first;
+          typeBuilder[i].descriptor(getTempHeapType(descriptor));
+          typeBuilder[i].setShared(descriptor.getShared());
           ++placeholderIt;
           return;
         }

test/lit/passes/gto-desc.wast

@@ -1111,6 +1111,68 @@
   )
 )
 
+;; Now we can optimize $A with a placeholder, but the placeholder must be
+;; shared.
+(module
+  (rec
+    ;; CHECK:      (rec
+    ;; CHECK-NEXT:  (type $0 (shared (descriptor $A.desc (struct))))
+
+    ;; CHECK:       (type $A (sub (shared (struct))))
+    (type $A (sub (shared (descriptor $A.desc (struct)))))
+    ;; CHECK:       (type $A.desc (sub (shared (describes $0 (struct)))))
+    (type $A.desc (sub (shared (describes $A (struct)))))
+
+    ;; CHECK:       (type $B (sub $A (shared (descriptor $B.desc (struct)))))
+    (type $B (sub $A (shared (descriptor $B.desc (struct)))))
+    ;; CHECK:       (type $B.desc (sub $A.desc (shared (describes $B (struct)))))
+    (type $B.desc (sub $A.desc (shared (describes $B (struct)))))
+  )
+
+  ;; CHECK:       (type $5 (func))
+
+  ;; CHECK:      (func $test (type $5)
+  ;; CHECK-NEXT:  (local $A (ref $A))
+  ;; CHECK-NEXT:  (local $A.desc (ref $A.desc))
+  ;; CHECK-NEXT:  (local $B (ref $B))
+  ;; CHECK-NEXT:  (local $B.desc (ref $B.desc))
+  ;; CHECK-NEXT:  (local.set $A
+  ;; CHECK-NEXT:   (struct.new_default $A)
+  ;; CHECK-NEXT:  )
+  ;; CHECK-NEXT:  (local.set $B
+  ;; CHECK-NEXT:   (struct.new_default $B
+  ;; CHECK-NEXT:    (struct.new_default $B.desc)
+  ;; CHECK-NEXT:   )
+  ;; CHECK-NEXT:  )
+  ;; CHECK-NEXT:  (drop
+  ;; CHECK-NEXT:   (ref.get_desc $B
+  ;; CHECK-NEXT:    (local.get $B)
+  ;; CHECK-NEXT:   )
+  ;; CHECK-NEXT:  )
+  ;; CHECK-NEXT: )
+  (func $test
+    (local $A (ref $A))
+    (local $A.desc (ref $A.desc))
+    (local $B (ref $B))
+    (local $B.desc (ref $B.desc))
+    (local.set $A
+      (struct.new $A
+        (struct.new $A.desc)
+      )
+    )
+    (local.set $B
+      (struct.new $B
+        (struct.new $B.desc)
+      )
+    )
+    (drop
+      (ref.get_desc $B
+        (local.get $B)
+      )
+    )
+  )
+)
+
 ;; Sibling types $A and $B. The supertype that connects them should not stop us
 ;; from optimizing $B here, even though $A cannot be optimized.
 (module