[llvm] [Hexagon/LoopIdiom] Protect test against O2 changes (PR #144734)

[Nikita Popov via llvm-commits]

https://github.com/nikic commented:

Is the table lookup better than the pmpy instructions? If not, then I don't think this change is right. You are hiding a legitimate regression.

https://github.com/llvm/llvm-project/pull/144734