[X86] Rename more target feature related things consistency. NFC

-Rename Mode*Bit to Is*Bit to match X86Subtarget.
-Rename FeatureLAHFSAHF to FeatureLAFHSAFH64 to match X86Subtarget.
-Use consistent capitalization

Reviewed By: skan

Differential Revision: https://reviews.llvm.org/D121975