[llvm] [InstCombine] Allow multi-use OptimizePointerDifference() with two GEPs (PR #90017)

[Nikita Popov via llvm-commits]

nikic wrote:

Compile-time: https://llvm-compile-time-tracker.com/compare.php?from=873889b7fa23434876533c603e687fc70396b0ab&to=402eea4442aaf4408e68d82aef10bae1f6a002aa&stat=instructions:u

No impact except improvements on stage2 builds, which means this results in better optimization.

https://github.com/llvm/llvm-project/pull/90017