[AArch64][AsmParser] Fix debug output in a few instructions

[c-rhodes]

Summary

In the parsing of BTIHint, PSBHint and Prefetch the identifier token
should be lexed after creating the operand, otherwise the StringRef is
moved before being copied and the debug output is incorrect.

Prefetch example:

$ echo "prfm   pldl1keep, [x2]" | ./bin/llvm-mc \
    -triple aarch64-none-linux-gnu -show-encoding -debug

Before:

  Matching formal operand class MCK_Prefetch against actual operand at
  index 1 (<prfop ,>): match success using generic matcher

After:

  Matching formal operand class MCK_Prefetch against actual operand at
  index 1 (<prfop pldl1keep>): match success using generic matcher

[david-arm]

LGTM!

llvm/lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp

AArch64AsmParser::tryParsePrefetch(OperandVector &Operands) {
   }
 
   auto PRFM = LookupByName(Tok.getString());
   if (!PRFM) {
     TokError("prefetch hint expected");
     return MatchOperand_ParseFail;
   }
 
-  Parser.Lex(); // Eat identifier token.
   Operands.push_back(AArch64Operand::CreatePrefetch(
       *PRFM, Tok.getString(), S, getContext()));
+  Parser.Lex(); // Eat identifier token.
   return MatchOperand_Success;
 }
 
 /// tryParsePSBHint - Try to parse a PSB operand, mapped to Hint command
 OperandMatchResultTy
 AArch64AsmParser::tryParsePSBHint(OperandVector &Operands) {
   MCAsmParser &Parser = getParser();
   SMLoc S = getLoc();
   const AsmToken &Tok = Parser.getTok();
   if (Tok.isNot(AsmToken::Identifier)) {
     TokError("invalid operand for instruction");
     return MatchOperand_ParseFail;
   }
 
   auto PSB = AArch64PSBHint::lookupPSBByName(Tok.getString());
   if (!PSB) {
     TokError("invalid operand for instruction");
     return MatchOperand_ParseFail;
   }
 
-  Parser.Lex(); // Eat identifier token.
   Operands.push_back(AArch64Operand::CreatePSBHint(
       PSB->Encoding, Tok.getString(), S, getContext()));
+  Parser.Lex(); // Eat identifier token.
   return MatchOperand_Success;
 }
 
 /// tryParseBTIHint - Try to parse a BTI operand, mapped to Hint command
 OperandMatchResultTy
 AArch64AsmParser::tryParseBTIHint(OperandVector &Operands) {
   MCAsmParser &Parser = getParser();
   SMLoc S = getLoc();
   const AsmToken &Tok = Parser.getTok();
   if (Tok.isNot(AsmToken::Identifier)) {
     TokError("invalid operand for instruction");
     return MatchOperand_ParseFail;
   }
 
   auto BTI = AArch64BTIHint::lookupBTIByName(Tok.getString());
   if (!BTI) {
     TokError("invalid operand for instruction");
     return MatchOperand_ParseFail;
   }
 
-  Parser.Lex(); // Eat identifier token.
   Operands.push_back(AArch64Operand::CreateBTIHint(
       BTI->Encoding, Tok.getString(), S, getContext()));
+  Parser.Lex(); // Eat identifier token.
   return MatchOperand_Success;
 }
 
 /// tryParseAdrpLabel - Parse and validate a source label for the ADRP
 /// instruction.
 OperandMatchResultTy
 AArch64AsmParser::tryParseAdrpLabel(OperandVector &Operands) {
   MCAsmParser &Parser = getParser();