Add sanity check in MLIR ODS to catch case where two operands have the same name

[mehdi_amini]

Summary

This is making a tablegen crash into a more friendly error.

mlir/tools/mlir-tblgen/OpDefinitionsGen.cpp

static void generateNamedOperandGetters(const Operator &op, Class &opClass,
   auto *m = opClass.addMethodAndPrune(rangeType, "getODSOperands", "unsigned",
                                       "index");
   auto &body = m->body();
   body << formatv(valueRangeReturnCode, rangeBeginCall,
                   "getODSOperandIndexAndLength(index)");
 
   // Then we emit nicer named getter methods by redirecting to the "sink" getter
   // method.
+  // Keep track of the operand names to find duplicates.
+  SmallDenseSet<StringRef> operandNames;
   for (int i = 0; i != numOperands; ++i) {
     const auto &operand = op.getOperand(i);
     if (operand.name.empty())
       continue;
+    if (!operandNames.insert(operand.name).second)
+      PrintFatalError(op.getLoc(), "op has two operands with the same name");
 
     if (operand.isOptional()) {
       m = opClass.addMethodAndPrune("::mlir::Value", operand.name);
       m->body()
           << "  auto operands = getODSOperands(" << i << ");\n"
           << "  return operands.empty() ? ::mlir::Value() : *operands.begin();";
     } else if (operand.isVariadicOfVariadic()) {
       StringRef segmentAttr =