[X86][SSE] Improve UINT_TO_FP v2i32 -> v2f64

Vectorize UINT_TO_FP v2i32 -> v2f64 instead of scalarization (albeit still on the SIMD unit).

The codegen matches that generated by legalization (and is in fact used by AVX for UINT_TO_FP v4i32 -> v4f64), but has to be done in the x86 backend to account for legalization via 4i32.

Differential Revision: https://reviews.llvm.org/D26938

llvm-svn: 287886