[X86] Rename FeatureCMPXCHG8B/FeatureCMPXCHG16B to FeatureCX8/CX16 to match CPUID.

Rename hasCMPXCHG16B() to canUseCMPXCHG16B() to make it less like other
feature functions. Add a similar canUseCMPXCHG8B() that aliases
hasCX8() to keep similar naming.

Differential Revision: https://reviews.llvm.org/D121978