[llvm] [AMDGPU] Exec-protect regular VGPR spill saves under narrowed exec (PR #184135)

[Michael Halkenhäuser via llvm-commits]

mhalk wrote:

For now this PR only includes functional changes.
Adding a focused lit test, as well as fixing affected, existing tests are WIP.

Please, let me know if this patch is deemed reasonable.

https://github.com/llvm/llvm-project/pull/184135