[llvm] [AMDGPU] Address follow-up concerns from 189121 (PR #192322)

[Jeffrey Byrnes via llvm-commits]

jrbyrnes wrote:

> @jrbyrnes This commit breaks some of the upstream buildbots and they have been red for a really long time, unless you have a fix ready to go it should be reverted.

It sounds like the issue is a bug in the llvm_unreachable implementation for certain compilers, and is independent from this NFC PR. I can take a look to help out, but I don't think we should be reverting unrelated NFC PRs to deal with random bugs for specific builds.

https://github.com/llvm/llvm-project/pull/192322