Subzero: Prototype to make use of RegNumT::No Register more concise

src/IceTargetLoweringX8664Traits.h

Index: src/IceTargetLoweringX8664Traits.h
diff --git a/src/IceTargetLoweringX8664Traits.h b/src/IceTargetLoweringX8664Traits.h
index 18e054ab9d295f1f376e1696247376c8a560a1b5..0ee1a690337c9ee8ef26d8586e7c1f93a6873483 100644
--- a/src/IceTargetLoweringX8664Traits.h
+++ b/src/IceTargetLoweringX8664Traits.h
@@ -401,7 +401,7 @@ private:
 
 public:
   static RegNumT getGprForType(Type Ty, RegNumT RegNum) {
-    assert(RegNum != RegNumT::NoRegister);
+    assert(RegNum.hasValue());
 
     if (!isScalarIntegerType(Ty)) {
       return RegNum;
@@ -730,14 +730,14 @@ public:
     static_assert(RegisterSet::Reg_xmm0 + 1 == RegisterSet::Reg_xmm1,
                   "Inconsistency between XMM register numbers and ordinals");
     if (ArgNum >= X86_MAX_XMM_ARGS) {
-      return RegNumT::NoRegister;
+      return RegNumT();
     }
     return RegNumT::fixme(RegisterSet::Reg_xmm0 + ArgNum);
   }
   /// Get the register for a given argument slot in the GPRs.
   static RegNumT getRegisterForGprArgNum(Type Ty, uint32_t ArgNum) {
     if (ArgNum >= X86_MAX_GPR_ARGS) {
-      return RegNumT::NoRegister;
+      return RegNumT();
     }
     static const RegisterSet::AllRegisters GprForArgNum[] = {
         RegisterSet::Reg_rdi, RegisterSet::Reg_rsi, RegisterSet::Reg_rdx,