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

[Graham Hunter via llvm-commits]

huntergr-arm wrote:

There's still several places where `getFixedValue()` is called where we might see a scalable immediate. I'll try writing some tests to see if I can reach them and fix them.

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