[turbofan] Index parameter to StoreWriteBarrier is word size.

R=jarin@chromium.org

Review URL: https://codereview.chromium.org/818173004

Cr-Commit-Position: refs/heads/master@{#26047}

src/compiler/arm64/code-generator-arm64.cc

@@ -720,7 +720,7 @@ void CodeGenerator::AssembleArchInstruction(Instruction* instr) {
       Register object = i.InputRegister(0);
       Register index = i.InputRegister(1);
       Register value = i.InputRegister(2);
-      __ Add(index, object, Operand(index, SXTW));
+      __ Add(index, object, index);
       __ Str(value, MemOperand(index));
       SaveFPRegsMode mode =
           frame()->DidAllocateDoubleRegisters() ? kSaveFPRegs : kDontSaveFPRegs;

src/compiler/x64/code-generator-x64.cc

@@ -962,7 +962,6 @@ void CodeGenerator::AssembleArchInstruction(Instruction* instr) {
       Register object = i.InputRegister(0);
       Register index = i.InputRegister(1);
       Register value = i.InputRegister(2);
-      __ movsxlq(index, index);
       __ movq(Operand(object, index, times_1, 0), value);
       __ leaq(index, Operand(object, index, times_1, 0));
       SaveFPRegsMode mode =