[AsmParser] Provide direct access to mnemonic token. Allow assignment parsing to be hooked by target. Allow target to specify if identifier is a label.

[colinl]

Summary

This allows AsmParsers to directly access the first token of a statement. This was creating an issue for statements that aren't lead by a token other than an identifier.

This also creates target hooks for checking if an identifier is a label and if AsmParser itself should parse assignment expressions.

"r31:30 = r21:20" is a register assignment. This trips up the parser because "r31:" looks like a label and an equals sign tells it to parse this as an assignment expression instead of an instruction.

[chandlerc]

And here is yet another commit that you landed without getting an LGTM from
anyone.

include/llvm/MC/MCTargetAsmParser.h

   /// \return True on failure.
   virtual bool ParseInstruction(ParseInstructionInfo &Info, StringRef Name,
                                 SMLoc NameLoc, OperandVector &Operands) = 0;
+  virtual bool ParseInstruction(ParseInstructionInfo &Info, StringRef Name,
+                                AsmToken Token, OperandVector &Operands) {
+    return ParseInstruction(Info, Name, Token.getLoc(), Operands);
+  }
 
   /// ParseDirective - Parse a target specific assembler directive
   ///

   virtual void convertToMapAndConstraints(unsigned Kind,
                                           const OperandVector &Operands) = 0;
 
+  // Return whether this parser uses assignment statements with equals tokens
+  virtual bool equalIsAsmAssignment() { return true; };
+  // Return whether this start of statement identifier is a label
+  virtual bool isLabel(AsmToken &Token) { return true; };
+
   virtual const MCExpr *applyModifierToExpr(const MCExpr *E,
                                             MCSymbolRefExpr::VariantKind,
                                             MCContext &Ctx) {

lib/MC/MCParser/AsmParser.cpp

   // See what kind of statement we have.
   switch (Lexer.getKind()) {
   case AsmToken::Colon: {
+    if (!getTargetParser().isLabel(ID))
+      break;
     checkForValidSection();
 
     // identifier ':'   -> Label.

   }
 
   case AsmToken::Equal:
+    if (!getTargetParser().equalIsAsmAssignment())
+      break;
     // identifier '=' ... -> assignment statement
     Lex();
 

   // Canonicalize the opcode to lower case.
   std::string OpcodeStr = IDVal.lower();
   ParseInstructionInfo IInfo(Info.AsmRewrites);
-  bool HadError = getTargetParser().ParseInstruction(IInfo, OpcodeStr, IDLoc,
+  bool HadError = getTargetParser().ParseInstruction(IInfo, OpcodeStr, ID,
                                                      Info.ParsedOperands);
   Info.ParseError = HadError;