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

[David Green via llvm-commits]

davemgreen 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.

I believe we do not use them any more because useFP16ConversionIntrinsics==false in clang.

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