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

[Amara Emerson via llvm-commits]

aemerson wrote:

> Oh, I see, reproduced.
> 
> Probably there should be a comment in this code that the base pointer is necessary to ensure the emergency spill slot is reachable.
> 
> Probably you can write a reduced testcase to force an emergency spill (just need to ensure you have a store to a stack slot with no scavengeable register... should be able to force that with inline asm). I'd be okay leaving it out if it turns out to be too tricky to write, though. The unreduced case probably isn't that useful.

I wasn't able to write a smaller test for the crash (probably did something wrong) but there was an existing SVE test for BP that I've replicated for SME.

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