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

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

> I had hoped the stack slot coloring would have removed the unnecessary slots, but that doesn't seem to happen (and would need a bit of work to support). Anyway, this LGTM

IIRC, the stack slot coloring depends on the lifetime markers (`LIFETIME_START/END`)

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