[llvm] NFC add a new precommit test case for PPCMIpeephole (PR #90656)

[Kamau Bridgeman via llvm-commits]

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

This test case LGTM, the CHECKs and CHECK-NOTs clearly show the peephole removing the extra sign extension. Thank you. Please update the PR description and, by extension, commit message to reference the issue this test case was pointed out in before committing.

https://github.com/llvm/llvm-project/pull/90656