[llvm] [NVPTX] Improve support for {ex2,lg2}.approx (PR #120519)

[Princeton Ferro via llvm-commits]

Prince781 wrote:

@AlexMaclean Tried with the `afn` flag on `@llvm.log2()`. This works only if you don't also have non-native operations that are expanded. e.g. `f16 = flog2 afn t0` will be expanded to `f16 = fptrunc (f32 flog2 (f32 fpextend t0))` where SelectionDAG drops `afn` on the new `f32 flog2` node, causing a crash.

It would be nice if SelectionDAG supported something like "preserve `afn`".

Anyway, I think these changes can be merged now.

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