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

[Krzysztof Parzyszek via llvm-commits]

kparzysz wrote:

> 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.

+1

I'd even venture a guess that polynomial multiplication is always better than a table lookup.

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