[llvm] [InstCombine] Make foldCmpLoadFromIndexedGlobal() GEP-type independent (PR #157089)

[Nikita Popov via llvm-commits]

nikic wrote:

> The implementation LGTM. The original logic for the state machines remains complicated. If possible, can you import some testcases from #67093?

Done. Some of the test cases are redundant after previous changes to this function (support for missing initial zero index was added separately), but I've imported the ones that looked valuable.

https://github.com/llvm/llvm-project/pull/157089