[llvm] [AMDGPU] Cost of i8 vector insert/extract is free in some cases (PR #194991)

[Janek van Oirschot via llvm-commits]

https://github.com/JanekvO approved this pull request.

Needs a rebase? LGTM otherwise
See if others have any remaining last comments

https://github.com/llvm/llvm-project/pull/194991