Fixing compile warnings for newer GCC versions due to unused variables.

src/x64/lithium-codegen-x64.cc

Index: src/x64/lithium-codegen-x64.cc
diff --git a/src/x64/lithium-codegen-x64.cc b/src/x64/lithium-codegen-x64.cc
index 49361e4f3468616243477995a988381245365429..e5050585278f0865025acb7cbe5d696bf0664b42 100644
--- a/src/x64/lithium-codegen-x64.cc
+++ b/src/x64/lithium-codegen-x64.cc
@@ -2246,7 +2246,6 @@ void LCodeGen::DoLoadKeyedFastElement(LLoadKeyedFastElement* instr) {
 
 void LCodeGen::DoLoadKeyedFastDoubleElement(
     LLoadKeyedFastDoubleElement* instr) {
-  Register elements = ToRegister(instr->elements());
   XMMRegister result(ToDoubleRegister(instr->result()));
 
   if (instr->hydrogen()->RequiresHoleCheck()) {
@@ -3101,7 +3100,6 @@ void LCodeGen::DoStoreKeyedFastElement(LStoreKeyedFastElement* instr) {
 void LCodeGen::DoStoreKeyedFastDoubleElement(
     LStoreKeyedFastDoubleElement* instr) {
   XMMRegister value = ToDoubleRegister(instr->value());
-  Register elements = ToRegister(instr->elements());
   Label have_value;
 
   __ ucomisd(value, value);