[mlir][PDLL] Allow numeric result indexing for unregistered op

If we don't specify the result index while matching operand with the
result of certain operation, it's supposed to match all the results of
the operation with the operand. For registered op, it's easy to do that
by either indexing with number or name. For unregistered op, this commit
enables the numeric result indexing for this use case.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D126330