[llvm] [ConstantFold] Fold `ilogb` and `ilogbf` when the input parameter is a constant value. (PR #113014)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> For libcalls like `ilogb` that do not have corresponding intrinsics, their intrinsic ID is 0. Therefore, we may still need to switch over the name for them.

Yes, there needs to be duplicated handling for intrinsics and libcalls (part of why I think most of the math intrinsics should be removed) 

https://github.com/llvm/llvm-project/pull/113014