[llvm] [SimplifyCFG] Relax `cttz` cost check in `simplifySwitchOfPowersOfTwo` (PR #145159)

[Nikita Popov via llvm-commits]

https://github.com/nikic approved this pull request.

LGTM

One thing this regressions is a power of two identity mapping (1 => 1, 2 => 2, 4 => 4, 8 => 8), which previously would just simplify to the input and now instead becomes cttz + table lookup. See https://github.com/dtcxzyw/llvm-opt-benchmark/pull/2486/files/7b32e72e21d0f2d7df01d737f7a43222df4b1b60#diff-469c9a904297396b18fb7e4fd311b56e92874a0c79a13d5f960f01304ec305de.

It probably make sense to not perform this transform in the early SimplifyCFG runs.

https://github.com/llvm/llvm-project/pull/145159