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

[Fangrui Song via llvm-commits]

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

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

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