Subzero: Refactor x86 register definitions to use the alias mechanism.

src/IceInst.h

Index: src/IceInst.h
diff --git a/src/IceInst.h b/src/IceInst.h
index abd814468db779a16c4ed48779faef45cb804d83..5e32904d470ac96be13b4d570178d6cbf218a7e5 100644
--- a/src/IceInst.h
+++ b/src/IceInst.h
@@ -382,8 +382,8 @@ public:
     /// Set HasSideEffects to true so that the call instruction can't be
     /// dead-code eliminated. IntrinsicCalls can override this if the particular
     /// intrinsic is deletable and has no side-effects.
-    const bool HasSideEffects = true;
-    const InstKind Kind = Inst::Call;
+    constexpr bool HasSideEffects = true;
+    constexpr InstKind Kind = Inst::Call;
     return new (Func->allocate<InstCall>()) InstCall(
         Func, NumArgs, Dest, CallTarget, HasTailCall, HasSideEffects, Kind);
   }