[TableGen] AsmMatcher: fix OpIdx of tied operands when HasOptionalOperands is true

[TOCK]

Summary

For tied operands, entry in ConversionTable does not store operand
index but the index of TiedAsmOperandTable where the actual operand
index comes from. Only subtract the number of optional operands after
actual operand index is known or it would use wrong entry from
TiedAsmOperandTable and tied to wrong operand as a result.

[TOCK]

Include full context.

llvm/utils/TableGen/AsmMatcherEmitter.cpp

emitConvertFuncs(CodeGenTarget &Target, StringRef ClassName,
   CvtOS << "    case CVT_Reg:\n";
   CvtOS << "      static_cast<" << TargetOperandClass
         << " &>(*Operands[OpIdx]).addRegOperands(Inst, 1);\n";
   CvtOS << "      break;\n";
   CvtOS << "    case CVT_Tied: {\n";
   CvtOS << "      assert(OpIdx < (size_t)(std::end(TiedAsmOperandTable) -\n";
   CvtOS << "                              std::begin(TiedAsmOperandTable)) &&\n";
   CvtOS << "             \"Tied operand not found\");\n";
-  CvtOS << "      unsigned TiedResOpnd = TiedAsmOperandTable[OpIdx][0];\n";
-  CvtOS << "      if (TiedResOpnd != (uint8_t)-1)\n";
+  CvtOS << "      unsigned TiedResOpnd = TiedAsmOperandTable[*(p + 1)][0];\n";
+  CvtOS << "      if (TiedResOpnd != (uint8_t)-1) {\n";
+  if (HasOptionalOperands)
+    CvtOS << "        TiedResOpnd -= DefaultsOffset[TiedResOpnd];\n";
   CvtOS << "        Inst.addOperand(Inst.getOperand(TiedResOpnd));\n";
+  CvtOS << "      };\n";
   CvtOS << "      break;\n";
   CvtOS << "    }\n";
 
   std::string OperandFnBody;
   raw_string_ostream OpOS(OperandFnBody);
   // Start the operand number lookup function.
   OpOS << "void " << Target.getName() << ClassName << "::\n"
        << "convertToMapAndConstraints(unsigned Kind,\n";