[llvm] [InstCombine] Fix multi-use handling for multi-GEP rewrite (PR #146689)

[Nikita Popov via llvm-commits]

nikic wrote:

@dtcxzyw I don't really have a schedule. My tentative next steps are going to be:

 * Land https://github.com/llvm/llvm-project/pull/137297. I think this is pretty much ready. I may delay this until after the LLVM 21 branch.
 * Split all multi-index GEPs (split off constant offsets).
 * Canonicalize scaling (possibly with an intermediate step of `[N x i8]`).

https://github.com/llvm/llvm-project/pull/146689