Add support to load/store byte fields.

src/x64/lithium-x64.cc

Index: src/x64/lithium-x64.cc
diff --git a/src/x64/lithium-x64.cc b/src/x64/lithium-x64.cc
index 5ee624f4b6fedd39b96b6c12634d05bd50e67ea1..7001c18f34ed129a6a5dc0d96b6b42c472071a0e 100644
--- a/src/x64/lithium-x64.cc
+++ b/src/x64/lithium-x64.cc
@@ -2068,7 +2068,14 @@ LInstruction* LChunkBuilder::DoStoreContextSlot(HStoreContextSlot* instr) {
 
 
 LInstruction* LChunkBuilder::DoLoadNamedField(HLoadNamedField* instr) {
-  if (instr->access().IsExternalMemory() && instr->access().offset() == 0) {
+  // Use the special mov rax, moffs64 encoding for external
+  // memory accesses with 64-bit word-sized values.
+  if (instr->access().IsExternalMemory() &&
+      instr->access().offset() == 0 &&
+      (instr->access().representation().IsSmi() ||
+       instr->access().representation().IsTagged() ||
+       instr->access().representation().IsHeapObject() ||
+       instr->access().representation().IsExternal())) {
     LOperand* obj = UseRegisterOrConstantAtStart(instr->object());
     return DefineFixed(new(zone()) LLoadNamedField(obj), rax);
   }