[llvm] [AArch64][SME] Vastly simplify and fix `sme-framelower-use-bp.ll` (NFC) (PR #172999)

[Sander de Smalen via llvm-commits]

https://github.com/sdesmalen-arm approved this pull request.

Strictly speaking it's not really checking the original intent that it can reach the emergency spill slot using the base-pointer, but now only checks that the base-pointer is used. It's still an improvement over the original test though.

https://github.com/llvm/llvm-project/pull/172999