[llvm] [SCEV] Try to re-use existing LCSSA phis when expanding SCEVAddRecExpr. (PR #147214)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

> Hm, this has no impact in the real world: [dtcxzyw/llvm-opt-benchmark#2550](https://github.com/dtcxzyw/llvm-opt-benchmark/pull/2550).

AFAIK `llvm-opt-benchmark` disables LoopVectorize, which is the main source of expansions where it could be beneficial.

Real-world code where this applies includes a few instances in `ffmpeg`, `brotli`, `freetype`, llvm's APFloat.cpp and a few others.

I will also share a follow-up patch that allows re-using existing LCSSA phi nodes in more cases.

https://github.com/llvm/llvm-project/pull/147214