X64: Add Crankshaft operation LoadGlobal.

src/x64/assembler-x64.cc

Index: src/x64/assembler-x64.cc
diff --git a/src/x64/assembler-x64.cc b/src/x64/assembler-x64.cc
index ef069a98bc1f5eb493347a5423efb73189cf20d6..699cb5b8d1906cb6cf0722a91990d701ba795a84 100644
--- a/src/x64/assembler-x64.cc
+++ b/src/x64/assembler-x64.cc
@@ -3076,10 +3076,15 @@ void Assembler::dd(uint32_t data) {
 void Assembler::RecordRelocInfo(RelocInfo::Mode rmode, intptr_t data) {
   ASSERT(rmode != RelocInfo::NONE);
   // Don't record external references unless the heap will be serialized.
-  if (rmode == RelocInfo::EXTERNAL_REFERENCE &&
-      !Serializer::enabled() &&
-      !FLAG_debug_code) {
-    return;
+  if (rmode == RelocInfo::EXTERNAL_REFERENCE) {
+#ifdef DEBUG
+    if (!Serializer::enabled()) {
+      Serializer::TooLateToEnableNow();
+    }
+#endif
+    if (!Serializer::enabled() && !FLAG_debug_code) {
+      return;
+    }
   }
   RelocInfo rinfo(pc_, rmode, data);
   reloc_info_writer.Write(&rinfo);