[LegalizeVectorOps][X86] Enable expansion of vector fp_to_uint in...

[LegalizeVectorOps][X86] Enable expansion of vector fp_to_uint in LegalizeVectorOps to avoid scalarization.

The code here isn't great in all caess. Particularly v4f64->v4i32
on 64-bit AVX targets. But there is some improvement in some
configurations.

There's definitely some issues with computeNumSignBits with
X86ISD::STRICT_FCMP. As well as not being able to propagate sign
bits through merge_values nodes that get created during custom
legalization.