[X86] Refactor movmsk(icmp_eq(and(x,c1),0)) -> movmsk(not(shl(x,c2))) fold to use KnownBits

We don't need an explicit AND mask, we can use KnownBits to determine if each element has (the same) single non-zero bit and shift that into the msb/signbit for MOVMSK to access directly.