[X86][SSE] LowerADDSAT_SUBSAT - avoid X86ISD::BLENDV in UADDSAT/USUBSAT custom lowering

Use the OR(CMP,ADD) / AND(CMP,SUB) patterns like we do on pre-SSE4 targets.

We're still using X86ISD::BLENDV on some AVX targets as we don't do custom lowering for >= 256-bit vectors.

Really this (and combineVSelectWithAllOnesOrZeros) needs moving to DAGCombiner, but pre-SSE42 we see the vXi64 comparison type as a 2 x 32-bits result so we can't just rely on ComputeNumSignBits to give us the 'all bits' result we need.