[llvm] [PassBuilder] Run trivial loop unswitching after late function-simpli… (PR #215899)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

>> This sounds like the first thing we should fix. It would eliminate the LICM part of the pass order dependence at pretty low cost. We may still need this change, but let's fix the simple thing and make sure.
> It's a two-level loop nest. 

As @preames mentioned, even though it won't handle the nested loop case, it is still something we can easily improve in `IndVarSimplify` and get some benefits end to end:

https://github.com/dtcxzyw/llvm-opt-benchmark-nightly/pull/1091

https://github.com/llvm/llvm-project/pull/215899