Subzero: Avoid explicit references to RegNumT sentinel value.

There are many occurrences of if (RegNum == RegNumT::NoRegister).

This patch eliminates NoRegister and provides a simpler mechanism for declaring and testing RegNumT values to see if they are undefined.

BUG= none
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1691193002 .

Patch from Reed Kotler <rkotlerimgtec@gmail.com>.

src/IceTargetLoweringMIPS32.h

diff --git a/src/IceTargetLoweringMIPS32.h b/src/IceTargetLoweringMIPS32.h
index 7608de1..08897e8 100644
--- a/src/IceTargetLoweringMIPS32.h
+++ b/src/IceTargetLoweringMIPS32.h
@@ -220,15 +220,15 @@
   };
   typedef uint32_t LegalMask;
   Operand *legalize(Operand *From, LegalMask Allowed = Legal_Default,
-                    RegNumT RegNum = RegNumT::NoRegister);
+                    RegNumT RegNum = RegNumT());
 
-  Variable *legalizeToVar(Operand *From, RegNumT RegNum = RegNumT::NoRegister);
+  Variable *legalizeToVar(Operand *From, RegNumT RegNum = RegNumT());
 
-  Variable *legalizeToReg(Operand *From, RegNumT RegNum = RegNumT::NoRegister);
+  Variable *legalizeToReg(Operand *From, RegNumT RegNum = RegNumT());
 
-  Variable *makeReg(Type Ty, RegNumT RegNum = RegNumT::NoRegister);
+  Variable *makeReg(Type Ty, RegNumT RegNum = RegNumT());
   static Type stackSlotType();
-  Variable *copyToReg(Operand *Src, RegNumT RegNum = RegNumT::NoRegister);
+  Variable *copyToReg(Operand *Src, RegNumT RegNum = RegNumT());
 
   void addProlog(CfgNode *Node) override;
   void addEpilog(CfgNode *Node) override;
@@ -240,7 +240,7 @@
   Operand *loOperand(Operand *Operand);
   Operand *hiOperand(Operand *Operand);
 
-  Operand *legalizeUndef(Operand *From, RegNumT RegNum = RegNumT::NoRegister);
+  Operand *legalizeUndef(Operand *From, RegNumT RegNum = RegNumT());
 
 protected:
   explicit TargetMIPS32(Cfg *Func);