[llvm] [NFC] update gfx12 vop test to use sed instead of grep (PR #120458)

[Wael Yehia via llvm-commits]

https://github.com/w2yehia approved this pull request.

Thanks @hubert-reinterpretcast for suggesting the sed solution. 
LGTM.

https://github.com/llvm/llvm-project/pull/120458