[llvm] [AMDGPU][GFX1250] Use null register for global_prefetch_b8 instead of s[0:1] (PR #214127)

[Shilei Tian via llvm-commits]

shiltian wrote:

The test failure actually is not caused by this PR, but instead a merge order. I have fixed it.

https://github.com/llvm/llvm-project/pull/214127