[mlir][Transform] Add a new navigation op to retrieve the producer of an operand

Given an opOperand uniquely determined by the operation `%op` and the operand number `num`,
the `transform.get_producer_of_operand %op[num]` returns the handle to the unique operation
that produced the SSA value used as opOperand.

The transform fails if the operand is a block argument.

Differential Revision: https://reviews.llvm.org/D134171