[llvm] [IR] Fix llvm.powi intrinsic decl (PR #216160)

[Paul Walker via llvm-commits]

paulwalker-arm wrote:

> newbie question: why not change the doc to match the ir?

It wouldn't be just a docs change because there's existing code that relies on the current specification (e.g. isVectorIntrinsicWithScalarOpAtArg). That doesn't stop the intrinsic being extended for the vector use case, it's just more work.

https://github.com/llvm/llvm-project/pull/216160