Tidy x86 assembler.

Use helper functions to compute when the kind has a SIB, a ModRM and RegReg
form.

Change-Id: I86a5cb944eec62451c63281265e6974cd7a08e07
diff --git a/compiler/dex/quick/arm64/assemble_arm64.cc b/compiler/dex/quick/arm64/assemble_arm64.cc
index 2c4f262..9362147 100644
--- a/compiler/dex/quick/arm64/assemble_arm64.cc
+++ b/compiler/dex/quick/arm64/assemble_arm64.cc
@@ -887,7 +887,7 @@
   CreateNativeGcMap();
 }
 
-int Arm64Mir2Lir::GetInsnSize(LIR* lir) {
+size_t Arm64Mir2Lir::GetInsnSize(LIR* lir) {
   ArmOpcode opcode = UNWIDE(lir->opcode);
   DCHECK(!IsPseudoLirOp(opcode));
   return EncodingMap[opcode].size;