[Codegen][GPU] Add fusion barrier after result promotion (#21709)

Currently result promotion relies on inserting two copies, one to
workgroup memory that is supposed to fuse with its producer, and one
with `derived_thread_config` that controls the distribution of the
consumer. This however has nothing forcing the intended fusion behavior,
and in all exercised cases today relies on reshapes missing fusion
patterns.

This patch introduces a fusion barrier op that sits between the two
copies to prevent fusion and gets dropped when bufferizing.