[X86] Prefer KORTEST on Knights Landing or later for memcmp()

PTEST and especially the MOVMSK instructions are slow on Knights Landing
or later. As a bonus, this patch increases instruction parallelism by
emitting:
    KORTEST(PCMPNEQ(a, b), PCMPNEQ(c, d)) == 0
Instead of:
    KORTEST(AND(PCMPEQ(a, b), PCMPEQ(c, d))) == ~0

https://reviews.llvm.org/D69157