Reduce field type tracking to a single stable map per field.

src/types.cc

Index: src/types.cc
diff --git a/src/types.cc b/src/types.cc
index c1225f396ec83985f57e64506d8718c7d3ecb2a1..37386cd82f6012e1ccf05caabb6e19fcd4b6664c 100644
--- a/src/types.cc
+++ b/src/types.cc
@@ -575,10 +575,7 @@ bool TypeImpl<Config>::NowIs(TypeImpl* that) {
 template<class Config>
 bool TypeImpl<Config>::NowStable() {
   DisallowHeapAllocation no_allocation;
-  for (Iterator<i::Map> it = this->Classes(); !it.Done(); it.Advance()) {
-    if (!it.Current()->is_stable()) return false;
-  }
-  return true;
+  return !this->IsClass() || this->AsClass()->Map()->is_stable();
 }