fix -Wunused-but-set-variable for gcc-4.6 on x64

src/x64/stub-cache-x64.cc

Index: src/x64/stub-cache-x64.cc
diff --git a/src/x64/stub-cache-x64.cc b/src/x64/stub-cache-x64.cc
index d4f9d7c20635bef4ae26bd1ea43f696b206761a1..b8e5f22ed548395afa1e51478334bbcb5cf51283 100644
--- a/src/x64/stub-cache-x64.cc
+++ b/src/x64/stub-cache-x64.cc
@@ -1089,9 +1089,8 @@ void StubCompiler::GenerateLoadConstant(JSObject* object,
   __ JumpIfSmi(receiver, miss);
 
   // Check that the maps haven't changed.
-  Register reg =
-      CheckPrototypes(object, receiver, holder,
-                      scratch1, scratch2, scratch3, name, miss);
+  CheckPrototypes(object, receiver, holder,
+                  scratch1, scratch2, scratch3, name, miss);
 
   // Return the constant value.
   __ Move(rax, Handle<Object>(value));
@@ -3746,8 +3745,6 @@ void KeyedStoreStubCompiler::GenerateStoreFastDoubleElement(
   __ bind(&is_nan);
   // Convert all NaNs to the same canonical NaN value when they are stored in
   // the double array.
-  ExternalReference canonical_nan_reference =
-      ExternalReference::address_of_canonical_non_hole_nan();
   __ Set(kScratchRegister, BitCast<uint64_t>(
       FixedDoubleArray::canonical_not_the_hole_nan_as_double()));
   __ movq(xmm0, kScratchRegister);