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

[Artem Belevich via llvm-commits]

Artem-B wrote:

Something odd is going on here. Skipping MVT::v8i8 here should've resulted in lowering extractelement using 64-bit BFE instruction. Yet we're ending with the code splitting 64-bit input it into two 32-bit words and then extracting bytes from each using 32-bit BFE. It's probably a wash performance-wise, as the actual registers are 32-bit, but it suggests we're missing something. We may need to mark the v8i8 extractelement legal.



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