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

[Joshua Cranmer via llvm-commits]

https://github.com/jcranmer-intel commented:

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.

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