[llvm] [AMDGPU] Preserve WWM ownership of allocated AGPRs (PR #223278)

[Yaxun Liu via llvm-commits]

yxsamliu wrote:

@krzysz00 Re: WWM copies

Some spilled SGPR values are stored in inactive vector lanes. `WWM_COPY` copies all lanes so those values survive a register copy, then restores `EXEC`.

https://github.com/llvm/llvm-project/pull/223278