[llvm] [LSR] Recognize vscale-relative immediates (PR #88124)

[Graham Hunter via llvm-commits]

huntergr-arm wrote:

Rebased, added some convenience functions.

I didn't switch to StackOffset yet. I think handling cases with mixed offsets requires more thought, and possibly redoing how we handle initial Use generation. When we do switch to it though, we can move the SCEV generation code to ScalarEvolution since that's an existing class available across the codebase.

https://github.com/llvm/llvm-project/pull/88124