Fix TypeMapper::getNewHeapType() on types being rebuilt (#8499)

When we find a type in the mapping, we still need to get its temp type,
as it may be in the process of being rebuilt.

Author: kripken

src/ir/type-updating.h

@@ -502,7 +502,7 @@ class TypeMapper : public GlobalTypeRewriter {
   HeapType getNewHeapType(HeapType type) {
     auto iter = mapping.find(type);
     if (iter != mapping.end()) {
-      return iter->second;
+      type = iter->second;
     }
     return getTempHeapType(type);
   }

test/lit/passes/type-merging-cont.wast

@@ -45,3 +45,40 @@
   )
 )
 
+;; A chain of continuations. During merging here we map types that are being
+;; rebuilt, and should not error while doing so. We can merge the function
+;; types, and all but two of the continuations.
+(module
+  (rec
+    ;; CHECK:      (rec
+    ;; CHECK-NEXT:  (type $func (sub (func)))
+    (type $func (sub (func)))
+    ;; CHECK:       (type $cont (sub (cont $func)))
+    (type $cont (sub (cont $func)))
+    (type $contB (sub $cont (cont $func)))
+    (type $contC (sub $contB (cont $func)))
+    (type $funcB (sub $func (func)))
+    ;; CHECK:       (type $contD (sub final $cont (cont $func)))
+    (type $contD (sub final $contC (cont $funcB)))
+  )
+
+  ;; CHECK:       (type $3 (func))
+
+  ;; CHECK:      (func $uses (type $3)
+  ;; CHECK-NEXT:  (local $func (ref $func))
+  ;; CHECK-NEXT:  (local $funcB (ref $func))
+  ;; CHECK-NEXT:  (local $cont (ref $cont))
+  ;; CHECK-NEXT:  (local $contB (ref $cont))
+  ;; CHECK-NEXT:  (local $contC (ref $cont))
+  ;; CHECK-NEXT:  (local $contD (ref $contD))
+  ;; CHECK-NEXT: )
+  (func $uses
+    (local $func (ref $func))
+    (local $funcB (ref $funcB))
+    (local $cont (ref $cont))
+    (local $contB (ref $contB))
+    (local $contC (ref $contC))
+    (local $contD (ref $contD))
+  )
+)
+