[MLIR][Vector] Add warp distribution for `scf.if` (#157119)

This PR adds `scf.if` op distribution to the existing `VectorDistribute`
patterns. The logic mostly follows that of `scf.for`: move op outside, wrap each
branch with `gpu.warp_execute_on_lane_0`. A notable difference to `scf.for` is
that each branch has its own set of escaping values, and `scf.if` itself does not
have block arguments.

GitOrigin-RevId: 7f007b572d63fe802a1f5587c74aae437f9f50e6