[llvm] [AArch64][SME] Reuse ZT0 spill slot (PR #158593)

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

> That's true for the StackColoring pass, but I thought the StackSlotColoring pass was able to do the same for spill slots without the markers.

Ah yeah, I think that one uses some "LiveStacks" analysis, which AFAIK is filled in by the register allocator, rather than a general analysis, so it only applies to spills produced during register allocation.  

https://github.com/llvm/llvm-project/pull/158593