[llvm] [AMDGPU] NFC: Add BBLiveOutMap & LiveOut Cache (PR #93089)

[Valery Pykhtin via llvm-commits]

vpykhtin wrote:

You've added two maps containing the same value but with different key. Should we only keep LiveOuts indexes by the region index? If not then maybe it worth to add a level of indirection and add a map of LastMIForRegion to RegionIdx or vice-versa? LiveRegSet is relatively heavy to copy and store and I would avoid that (this is a deep copy not the pointer copy).

https://github.com/llvm/llvm-project/pull/93089