[turbofan] Make global variable loads and stores explicit.

src/types.cc

Index: src/types.cc
diff --git a/src/types.cc b/src/types.cc
index 18796151cc71c306152ed8e156b350eeb9ece02d..1c6b84e2dc67da07ce3c809810b163bf306e2ca1 100644
--- a/src/types.cc
+++ b/src/types.cc
@@ -235,6 +235,7 @@ TypeImpl<Config>::BitsetType::Lub(i::Map* map) {
     case JS_GENERATOR_OBJECT_TYPE:
     case JS_MODULE_TYPE:
     case JS_BUILTINS_OBJECT_TYPE:
+    case JS_GLOBAL_OBJECT_TYPE:
     case JS_GLOBAL_PROXY_TYPE:
     case JS_ARRAY_BUFFER_TYPE:
     case JS_ARRAY_TYPE:
@@ -248,8 +249,6 @@ TypeImpl<Config>::BitsetType::Lub(i::Map* map) {
     case JS_WEAK_SET_TYPE:
       if (map->is_undetectable()) return kUndetectable;
       return kOtherObject;
-    case JS_GLOBAL_OBJECT_TYPE:
-      return kGlobalObject;
     case JS_FUNCTION_TYPE:
       return kOtherObject;  // TODO(rossberg): there should be a Function type.
     case JS_REGEXP_TYPE: