[X86] Don't copy kill flag when expanding LCMPXCHG16B_SAVE_RBX

The expansion code creates a copy to RBX before the real LCMPXCHG16B.
It's possible this copy uses a register that is also used by the
real LCMPXCHG16B. If we set the kill flag on the use in the copy,
then we'll fail the machine verifier on the use on the LCMPXCHG16B.

Differential Revision: https://reviews.llvm.org/D89151