[llvm] test: check const-fold-gep with poison, undef, id (PR #125573)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

I looked at 1383cb6 ([ConstFold] Fix incorrect gep inbounds of undef fold), and it only seems to be tested indirectly in InstSimplify/ConstProp. As far as I can tell, there is no direct test, but I fully understand if you don't want to extend a non-UTC test.

https://github.com/llvm/llvm-project/pull/125573