[mlir] Transform dialect: introduce merge_handles op

This Transform dialect op allows one to merge the lists of Payload IR
operations pointed to by several handles into a single list associated with one
handle. This is an important Transform dialect usability improvement for cases
where transformations may temporarily diverge for different groups of Payload
IR ops before converging back to the same script. Without this op, several
copies of the trailing transformations would have to be present in the
transformation script.

Depends On D129090

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D129110