[llvm] Use Log2_64_Ceil to compute PowerOf2Ceil (PR #67580)

[via llvm-commits]

AtariDreams wrote:

> This is manual strength reduction. `NextPowerOf2` is a complex operation whose original purpose isn't clear to me.

The only thing NextPowerOf2 has doing for it is that it is constexpr but also like it's used in non constexpr places too but that is something for another PR to address

https://github.com/llvm/llvm-project/pull/67580