[llvm] [InstCombine] Eliminate fptrunc/fpext if fast math flags allow it (PR #115027)

[Nikita Popov via llvm-commits]

nikic wrote:

> > I'd much rather we bite the bullet and add the fast-math flags onto the FP conversion operations, rather than trying to play game checking the users to figure out what their fast-math flags imply the conversion operation's flags should be.
> 
> Sounds sensible. I'll get to work on that then adjust this patch to check the trunc/ext.

For reference, there's an RFC for that here: https://discourse.llvm.org/t/rfc-fmf-on-more-instructions/82978

https://github.com/llvm/llvm-project/pull/115027