[llvm] [SelectionDAG] Remove `UnsafeFPMath` in `visitFP_ROUND` (PR #154768)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm approved this pull request.

LGTM. I wish the "N0IsTrunc" operand was a real fast math flag we could directly test against 

https://github.com/llvm/llvm-project/pull/154768