[mlir] optionally allow repeated handles in transform dialect

Some operations may be able to deal with handles pointing to the same
operation when the handle is consumed. For example, merge handles with
deduplication doesn't actually destroy payload operations and is
specifically intended to remove the situation with duplicates. Add a
method to the transform interface to allow ops to declare they can
support repeated handles.

Reviewed By: springerm

Differential Revision: https://reviews.llvm.org/D140124