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_x86.h b/compiler/optimizing/code_generator_x86.h
index 8252f81..0aff6cc 100644
--- a/compiler/optimizing/code_generator_x86.h
+++ b/compiler/optimizing/code_generator_x86.h
@@ -25,7 +25,8 @@
 namespace art {
 namespace x86 {
 
-static constexpr size_t kX86WordSize = 4;
+// Use a local definition to prevent copying mistakes.
+static constexpr size_t kX86WordSize = kX86PointerSize;
 
 class CodeGeneratorX86;
 class SlowPathCodeX86;