[llvm] [LV] Consolidate shouldOptimizeForSize and remove unused BFI/PSI. NFC (PR #168697)

[Luke Lau via llvm-commits]

lukel97 wrote:

@artagnon sorry for the post LGTM commit, but I was a bit confused about what OptForSize was doing in LoopVectorizationLegality. Turns out it's because we don't want to emit SCEV checks with OptForSize enabled, see https://reviews.llvm.org/D68082, so I've pushed a change to rename it

https://github.com/llvm/llvm-project/pull/168697