[llvm] [InstCombine] Always rewrite multi-use GEP for pointer difference (PR #142787)

[Nikita Popov via llvm-commits]

nikic wrote:

> I plan to extend this code to deeper GEP chains, in which case always rewriting seems like a good idea.

Based on the llvm-opt-benchmark results for https://github.com/llvm/llvm-project/pull/142958 this doesn't seem to be strictly necessary in practice. So I've made that change independentl from this one. The llvm-opt-benchmark diffs for this one look fairly neutral overall, but it's a bit hard to judge with this kind of change.

https://github.com/llvm/llvm-project/pull/142787