[X86] Allow v2i32->v2f32 strict and non-strict uint_to_fp to be widened to...

[X86] Allow v2i32->v2f32 strict and non-strict uint_to_fp to be widened to v4i32->v4f32 under avx512.

With avx512vl we get v4i32->v4f32 uint_to_fp instructions. With
avx512f we get v16i32->v16f32 instructions which we can use to
emulate v4i32->v4f32.