[llvm] [NVPTX] Support llvm.exp2 for f32 and vector of f32 (PR #120519)

[Princeton Ferro via llvm-commits]

Prince781 wrote:

> I'm not sure that lowering fexp2 to `ex2.approx` is a good idea.
> 
> At the very least it should've been conditional to some sort of fast math flag allowing reduced precision.

I think it's not a bad idea since there is no non-approximate implementation in PTX, which is something users of NVPTX should know. Making the lowering only work for fast-math would break unoptimized code.

https://github.com/llvm/llvm-project/pull/120519