[Custom Descriptors] Optimize descriptors in Unsubtyping

With the upcoming stricter validation rules for descriptor types, we can
no longer use the placeholder describee trick in GlobalTypeOptimization,
so its ability to optimize out unneeded descriptors will be
significantly restricted. Because of the increasing coupling between the
validation for subtyping and descriptor relationships, it makes most
sense to optimize both at the same time in Unsubtyping.

Update Unsubtyping to start the analysis assuming no descriptors are
necessary, then add them in as it discovers that they are indeed
necessary either to preserve behavior or validitiy. Discovering new
required descriptors can imply that new subtypings are necessary and
vice versa.

Author: tlively

src/ir/subtype-exprs.h

@@ -311,8 +311,16 @@ struct SubtypingDiscoverer : public OverriddenVisitor<SubType> {
   void visitRefCast(RefCast* curr) { self()->noteCast(curr->ref, curr); }
   void visitRefGetDesc(RefGetDesc* curr) {}
   void visitBrOn(BrOn* curr) {
-    if (curr->op == BrOnCast || curr->op == BrOnCastFail) {
-      self()->noteCast(curr->ref, curr->castType);
+    switch (curr->op) {
+      case BrOnNull:
+      case BrOnNonNull:
+        break;
+      case BrOnCast:
+      case BrOnCastFail:
+      case BrOnCastDesc:
+      case BrOnCastDescFail:
+        self()->noteCast(curr->ref, curr->castType);
+        break;
     }
     self()->noteSubtype(curr->getSentType(),
                         self()->findBreakTarget(curr->name));