[llvm] [InstCombine] Increase coverage of shuffle-gep (PR #139725)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

> LGTM. Might be worth checking flag preservation as well?

Yes, the flag preservation is only for inbounds at the moment. I have a follow-up fixing that with tests, downstream.

https://github.com/llvm/llvm-project/pull/139725