[X86] LowerVSELECT - split v16i16/v32i8 pre-AVX2 VSELECT ops if enough of the...

[X86] LowerVSELECT - split v16i16/v32i8 pre-AVX2 VSELECT ops if enough of the operands are free to split.

Often on AVX1 we're better off consistently using 128-bit instructions, so recognise when the operands are loads that can be freely/cheaply split - ideally this functionality needs to be moved to isFreeToSplitVector but we're using it in a few places where we don't want to split loads yet.

Based off a regression reported after #92794