[llvm] [AArch64][SME] Fix accessing the emergency spill slot with hazard padding (PR #142190)

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

> Instead of forcing a base pointer, we could relocate the emergency spill slot so it's part of the callee-save region. But I guess if you do that's, it's expensive to refer to local variables, even if it's still technically possible.

Hmm, that'd probably require a little more rework, and given the base pointer is likely to be beneficial in this case, I don't feel the need to avoid it. 

https://github.com/llvm/llvm-project/pull/142190