[x64] Process the B field of the REX prefix correctly for the PUSH and POP

instructions

Summary: This patch makes `x86AssemblyInspectionEngine` to process zero value of
the `B` field of the `REX` prefix in a correct way for `PUSH` and `POP`
instructions. MSVC sometimes emits `pushq %rbp` instruction as `0x40 0x55`, and
it was not parsed correctly before.

Reviewers: jasonmolenda, labath

Reviewed By: jasonmolenda, labath

Subscribers: abidh, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D57745

llvm-svn: 353281