[X86] combineSelect - don't constant fold BLENDV nodes like VSELECT

If a X86ISD::BLENDV op appears before legalization (in this test case due to the icmp_slt x, 0) its constant mask was being treated as a vselect mask (mask != 0) instead of blendv (mask < 0)

This just prevents constant folding entirely for non-VSELECT ops.