[MLIR] Improve op parse error message for AtLeastNOperands trait

[bondhugula]

Summary

Improve parse error message for "at least N operands" op trait.

[lattner]

nice!

[rriddle]

Thanks for this!

mlir/lib/IR/Operation.cpp

LogicalResult OpTrait::impl::verifyNOperands(Operation *op,
   }
   return success();
 }
 
 LogicalResult OpTrait::impl::verifyAtLeastNOperands(Operation *op,
                                                     unsigned numOperands) {
   if (op->getNumOperands() < numOperands)
     return op->emitOpError()
-           << "expected " << numOperands << " or more operands";
+           << "expected " << numOperands << " or more operands, but found "
+           << op->getNumOperands();
   return success();
 }
 
 /// If this is a vector type, or a tensor type, return the scalar element type
 /// that it is built around, otherwise return the type unmodified.
 static Type getTensorOrVectorElementType(Type type) {
   if (auto vec = type.dyn_cast<VectorType>())
     return vec.getElementType();