[llvm] [ConstantFolding] Add ilogb in isMathLibCallNoop (PR #122582)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Thanks, I will surely make this change in another patch, can you share your WIP implementation if possible? I want to make more contributions and learn more :)

It's not really in a postable state (and I kind of forgot about, it's many months old) 

https://github.com/llvm/llvm-project/pull/122582