[llvm] [SelectionDAG] Remove `UnsafeFPMath` in LegalizeDAG (PR #146316)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Currently I couldn't fix the test `CodeGen/ARM/fp16.ll`, because `llvm.convert.to.fp16.f64` in this test is incompatible with fast math flags.

Just take the regressions, these intrinsics should be removed from the IR 

https://github.com/llvm/llvm-project/pull/146316