[llvm] Do not use PerformEXTRACTCombine for v8i8 types (PR #81242)

[Artem Belevich via llvm-commits]

https://github.com/Artem-B approved this pull request.

LGTM. Puzzling changes in PTX output are still valid, and the test fixes are useful.
I'll take a look at the unexpected lowering separately.

https://github.com/llvm/llvm-project/pull/81242