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

[Amara Emerson via llvm-commits]

aemerson 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 .)

The test case I have is a pretty large one that causes a crash. I'll add it but that was the only motivation for this, not to improve code.

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