[llvm] [AggressiveInstCombine] Avoid converting first table element when folding table-based cttz (PR #211202)

[Antonio Frighetto via llvm-commits]

antoniofrighetto wrote:

It may be possible that the other log2 pattern has similar issues, though couldn't craft a test for that (despite the TTI cost check).

https://github.com/llvm/llvm-project/pull/211202