[llvm] [AArch64][SME] Fix frame lowering not using a base pointer for SME functions. (PR #91643)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

Please add a testcase where a base pointer actually improves the generated code.  None of the testcases you're modifying even materialize the base pointer.

(I think some of the existing testcases are likely to be changed to add +sve; see discussion on #83301 .)

https://github.com/llvm/llvm-project/pull/91643