[GC] Fully implement RTT semantics (#3441)

This adds info to RTT literals so that they can represent the chain of
rtt.canon/sub commands that generated them, and it adds an internal
RTT for each GC allocation (array or struct).

The approach taken is to simply store the full chain of rtt.sub types
that led to each literal. This is not efficient, but it is simple and seems
sufficient for the semantics described in the GC MVP doc - specifically,
only the types matter, in that repeated executions of rtt.canon/sub
on the same inputs yield equal outputs.

This PR fixes a bunch of minor issues regarding that, enough to allow testing
of the optimization and execution of ref.test/cast.

1 files changed, 3 insertions, 2 deletions

diff --git a/src/ir/global-utils.h b/src/ir/global-utils.h
index 7dc4c6af3..053eb0456 100644
--- a/src/ir/global-utils.h
+++ b/src/ir/global-utils.h
@@ -56,13 +56,14 @@ getGlobalInitializedToImport(Module& wasm, Name module, Name base) {
 inline bool canInitializeGlobal(const Expression* curr) {
   if (auto* tuple = curr->dynCast<TupleMake>()) {
     for (auto* op : tuple->operands) {
-      if (!Properties::isSingleConstantExpression(op) && !op->is<GlobalGet>()) {
+      if (!canInitializeGlobal(op)) {
         return false;
       }
     }
     return true;
   }
-  return Properties::isSingleConstantExpression(curr) || curr->is<GlobalGet>();
+  return Properties::isSingleConstantExpression(curr) ||
+         curr->is<GlobalGet>() || curr->is<RttCanon>() || curr->is<RttSub>();
 }
 
 } // namespace GlobalUtils