[MLIR] Add a comment to `requiredOperandCount` arg in LoopOps.cpp.

[rriddle]

Thanks Alex! Feel free to submit really small things like this directly next time.

mlir/lib/Dialect/LoopOps/LoopOps.cpp

if (parser.parseKeyword("step") ||
       parser.parseOperandList(steps, ivs.size(),
                               OpAsmParser::Delimiter::Paren) ||
       parser.resolveOperands(steps, builder.getIndexType(), result.operands))
     return failure();
 
   // Parse step value.
   SmallVector<OpAsmParser::OperandType, 4> initVals;
   if (succeeded(parser.parseOptionalKeyword("init"))) {
-    if (parser.parseOperandList(initVals, -1, OpAsmParser::Delimiter::Paren))
+    if (parser.parseOperandList(initVals, /*requiredOperandCount=*/-1,
+                                OpAsmParser::Delimiter::Paren))
       return failure();
   }
 
   // Now parse the body.
   Region *body = result.addRegion();
   SmallVector<Type, 4> types(ivs.size(), builder.getIndexType());
   if (parser.parseRegion(*body, ivs, types))
     return failure();