[RISCV][NFC] Use NoRegister instead of 0 literal

[luismarques]

Summary

Trivial cleanup.

[lenary]

LGTM

[luismarques]

Rebased on master.

[grimar]

It broke BB: http://lab.llvm.org:8011/builders/llvm-clang-x86_64-expensive-checks-win/builds/19706/steps/build-unified-tree/logs/stdio

llvm/trunk/lib/Target/RISCV/AsmParser/RISCVAsmParser.cpp

 
 // Attempts to match Name as a register (either using the default name or
 // alternative ABI names), setting RegNo to the matching register. Upon
 // failure, returns true and sets RegNo to 0. If IsRV32E then registers
 // x16-x31 will be rejected.
 static bool matchRegisterNameHelper(bool IsRV32E, Register &RegNo,
                                     StringRef Name) {
   RegNo = MatchRegisterName(Name);
-  if (RegNo == 0)
+  if (RegNo == NoRegister)
     RegNo = MatchRegisterAltName(Name);
   if (IsRV32E && RegNo >= RISCV::X16 && RegNo <= RISCV::X31)
-    RegNo = 0;
-  return RegNo == 0;
+    RegNo = NoRegister;
+  return RegNo == NoRegister;
 }
 
 bool RISCVAsmParser::ParseRegister(unsigned &RegNo, SMLoc &StartLoc,
                                    SMLoc &EndLoc) {
   const AsmToken &Tok = getParser().getTok();
   StartLoc = Tok.getLoc();
   EndLoc = Tok.getEndLoc();
   RegNo = 0;
default:
     if (HadParens)
       getLexer().UnLex(LParen);
     return MatchOperand_NoMatch;
   case AsmToken::Identifier:
     StringRef Name = getLexer().getTok().getIdentifier();
     Register RegNo;
     matchRegisterNameHelper(isRV32E(), RegNo, Name);
 
-    if (RegNo == 0) {
+    if (RegNo == NoRegister) {
       if (HadParens)
         getLexer().UnLex(LParen);
       return MatchOperand_NoMatch;
     }
     if (HadParens)
       Operands.push_back(RISCVOperand::createToken("(", FirstS, isRV64()));
     SMLoc S = getLoc();
     SMLoc E = SMLoc::getFromPointer(S.getPointer() - 1);