[llvm] [GlobalOpt] Precommit tests for PR84694 (PR #87443)

[Anshil Gandhi via llvm-commits]

gandhi56 wrote:

> Thanks for splitting off the tests. Would it make sense to have them all in a single file?
> 
> Also would be good to add a prefix like `[GlobalOpt]` to the title.

Done, I figured it would be best to add these tests in the existing test file specific to gep and constexpr operators.

https://github.com/llvm/llvm-project/pull/87443