[X86] Make CMPXCHG16B feature imply CMPXCHG8B feature.

This fixes znver1 so that it properly enables CMPXHG8B. We can
probably remove explicit CMPXCHG8B from CPUs that also have
CMPXCHG16B, but keeping this simple to allow cherry pick to 9.0.

Fixes PR42935.

llvm-svn: 368324