[RISCV] Fix parseBareSymbol to not double-parse top-level operators

[jrtc27]

Summary

By failing to lex the token we end up both parsing it as a binary
operator ourselves and parsing it as a unary operator when calling
parseExpression on the RHS. For plus this is harmless but for minus this
parses "foo - 4" as "foo - -4", effectively treating a top-level minus
as a plus.

Fixes https://github.com/llvm/llvm-project/issues/54105

[MaskRay]

Looks great!

llvm/test/MC/RISCV/rvi-pseudos.s

191

End a full sentence with a period.

[asb]

Thanks for the fix! LGTM.

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

OperandMatchResultTy RISCVAsmParser::parseBareSymbol(OperandVector &Operands) {
 
   MCBinaryExpr::Opcode Opcode;
   switch (getLexer().getKind()) {
   default:
     Operands.push_back(RISCVOperand::createImm(Res, S, E, isRV64()));
     return MatchOperand_Success;
   case AsmToken::Plus:
     Opcode = MCBinaryExpr::Add;
+    getLexer().Lex();
     break;
   case AsmToken::Minus:
     Opcode = MCBinaryExpr::Sub;
+    getLexer().Lex();
     break;
   }
 
   const MCExpr *Expr;
   if (getParser().parseExpression(Expr, E))
     return MatchOperand_ParseFail;
   Res = MCBinaryExpr::create(Opcode, Res, Expr, getContext());
   Operands.push_back(RISCVOperand::createImm(Res, S, E, isRV64()));

llvm/test/MC/RISCV/rvi-pseudos.s

 # CHECK: sw  a3, %pcrel_lo(.Lpcrel_hi29)(a4)
 sw a3, zero, a4
 
 ## Check that a complex expression can be simplified and matched.
 # CHECK: .Lpcrel_hi30:
 # CHECK: auipc a5, %pcrel_hi((255+a_symbol)-4)
 # CHECK: addi  a5, a5, %pcrel_lo(.Lpcrel_hi30)
 lla a5, (0xFF + a_symbol) - 4
+
+## Check that we don't double-parse a top-level minus.
+# CHECK: .Lpcrel_hi31:
+# CHECK: auipc a5, %pcrel_hi(a_symbol-4)
+# CHECK: addi  a5, a5, %pcrel_lo(.Lpcrel_hi31)
+lla a5, a_symbol - 4