[llvm] [flang] [clang] [InstCombine] Canonicalize constant GEPs to i8 source element type (PR #68882)

[Arthur Eubanks via cfe-commits]

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

I think this is a good direction to go, lgtm

https://github.com/llvm/llvm-project/pull/68882