[llvm] [MISched] Use SchedRegion in overrideSchedPolicy and overridePostRASchedPolicy (PR #149297)

[Pengcheng Wang via llvm-commits]

https://github.com/wangpc-pp approved this pull request.

LGTM.

(We can also change `enterRegion` but this can be a follow-up)

https://github.com/llvm/llvm-project/pull/149297