[clang] [llvm] [IR] Allow fast math flags on fptosi, fptoui and sitofp (PR #160475)

[via llvm-commits]

paperchalice wrote:

> I don't remember the example off-hand, but there was a fast-math transform I wanted at one point that needed a flag on `fptoui`.
> 
> > I don't think we should do this until we have freed up the necessary flag space to also support uitofp as well.
> 
> I can see an argument for doing `fptoui` and to `fptosi` without the inverses, but agreed that `sitofp` without `uitofp` is a bad idea.

PowerPC also need `sitofp/uitofp` with `afn` to allow double rounding.

https://github.com/llvm/llvm-project/pull/160475