Consistently use k{InstructionSet}WordSize.

These constants were defined prior to k{InstructionSet}PointerSize. So
use them consistently in optimizing as a first step. We can discuss
whether we should remove them in a second step.

Change-Id: If129de1a3bb8b65f8d9c816a8ad466815fb202e6
diff --git a/compiler/optimizing/code_generator_arm64.h b/compiler/optimizing/code_generator_arm64.h
index 6b71b94..a40f27f 100644
--- a/compiler/optimizing/code_generator_arm64.h
+++ b/compiler/optimizing/code_generator_arm64.h
@@ -31,7 +31,9 @@
 class CodeGeneratorARM64;
 class SlowPathCodeARM64;
 
-static constexpr size_t kArm64WordSize = 8;
+// Use a local definition to prevent copying mistakes.
+static constexpr size_t kArm64WordSize = kArm64PointerSize;
+
 static const vixl::Register kParameterCoreRegisters[] = {
   vixl::x1, vixl::x2, vixl::x3, vixl::x4, vixl::x5, vixl::x6, vixl::x7
 };