[X86] Remove X86ISD::LCMPXCHG8_SAVE_EBX_DAG and LCMPXCHG8B_SAVE_EBX pseudo instruction

This and its friend X86ISD::LCMPXCHG8_SAVE_RBX_DAG are used if we need to avoid clobbering the frame pointer in EBX/RBX. EBX/RBX are only used a frame pointer in 64-bit mode. In 64-bit mode we don't use CMPXCHG8B since we have a GR64 cmpxchg available. So we don't need special handling for LCMPXCHG8B.

Split from D88808

Differential Revision: https://reviews.llvm.org/D88853