[mlir][gpu] Add `subgroup_reduce` operation

Introduce `subgroup_reduce` operation, similar to `all_reduce`, but operating on subgroup scope instead of workgroup.
It is intended as low-level building block for more high level abstractions (e.g for workgroup-wide `all_reduce` ops).
Only introduce version taking reduce operation enum for simplicity sake.

Differential Revision: https://reviews.llvm.org/D135323