[X86] Relax restrictions on what setcc condition codes can be folded with a...

[X86] Relax restrictions on what setcc condition codes can be folded with a sext when AVX512 is enabled.

We now allow all signed comparisons and not equal. The complement that needs to be added for this is no worse than the extend. And the vector output forms of pcmpeq/pcmpgt have better latency than the k-register version on SKX.

llvm-svn: 324294