[X86][AVX] LowerADDSAT_SUBSAT - avoid X86ISD::BLENDV in UADDSAT/USUBSAT v8i32/v4i64 lowering

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

Enable custom lowering for v8i32/v4i64 and generalize the 128-bit lowering code for any vector size - this also lets us use the slightly cheaper codegen for icmp_ugt instead of umin/umax.