[X86] Handle splats in LowerBUILD_VECTORvXi1 by directly emitting scalar...

[X86] Handle splats in LowerBUILD_VECTORvXi1 by directly emitting scalar selects instead of deferring that to LowerSELECT.

LoweSELECT will detect the constant inputs and convert to scalar
selects, but we can do it directly here.

I might remove some of the code from LowerSELECT and move it to
DAG combine so doing this explicitly will make us less dependent
on it happening in lowering.