[llvm] [AMDGPU] Split vgpr regalloc pipeline (PR #93526)

[Yaxun Liu via llvm-commits]

yxsamliu wrote:

> LLVM Buildbot has detected a new failure on builder `lld-x86_64-win` running on `as-worker-93` while building `llvm` at step 7 "test-build-unified-tree-check-all".
> 
> Full details are available at: https://lab.llvm.org/buildbot/#/builders/146/builds/1277
> 
> Here is the relevant piece of the build log for the reference

The failure was due to blender perf degradation about 10%. Since this patch is critical for correctness, we won't revert this patch due to perf degradation. Instead, we will address it with following up fix. Currently this failure was silenced by rebasing the perf results in the buildbot.

https://github.com/llvm/llvm-project/pull/93526