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

[John Brawn via llvm-commits]

john-brawn-arm 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.

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