[turbofan] Index parameter to StoreWriteBarrier is word size.

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

Index: src/compiler/arm64/code-generator-arm64.cc
diff --git a/src/compiler/arm64/code-generator-arm64.cc b/src/compiler/arm64/code-generator-arm64.cc
index 5bb9ee6a6b525ed72f9a6ca5a177fa0ea3bf9e82..ecdf148cab5c6516d96231a1925ec56f675e322e 100644
--- a/src/compiler/arm64/code-generator-arm64.cc
+++ b/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;