[X86] Improve v2i64->v2f32 and v4i64->v4f32 uint_to_fp on avx and avx2 targets.

Summary:
Based on Simon's D52965, but improved to handle strict fp and improve some of the shuffling.

Rather than use v2i1/v4i1 and let type legalization continue, just generate all the code with legal types and use an explicit shuffle.

I also added an explicit setcc to the v4i64 code to match the semantics of vselect which doesn't just use the sign bit. I'm also using a v4i64->v4i32 truncate instead of the shuffle in Simon's original code. With the setcc this will become a pack.

Future work can look into using X86ISD::BLENDV and a different shuffle that only moves the sign bit.

Reviewers: RKSimon, spatel

Reviewed By: RKSimon

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71956