[llvm] [TargetLowering][SelectionDAG] Exploit nneg Flag in UINT_TO_FP (PR #108931)

[Michael Marjieh via llvm-commits]

mmarjieh wrote:

@arsenm Can you submit it on my behalf? I don't have write access.

**Squashed Commit Message:**
[TargetLowering][SelectionDAG] Exploit nneg Flag in UINT_TO_FP 

Propogate the nneg flag in WidenVecRes
Use SINT_TO_FP in expandUINT_TO_FP when possible.

https://github.com/llvm/llvm-project/pull/108931