[llvm] [AMDGPU] Use global_prefetch_b8 for GFX1250 unclaused VMEM workaround (PR #210874)

[Siu Chi Chan via llvm-commits]

scchan wrote:

> Do you plan to update the assembler warning part of this from #209517? They should be kept consistent right?

Yes I'll work on that

https://github.com/llvm/llvm-project/pull/210874