[X86] Do not use cmpxchgXXb when we need the base pointer (RBX).

cmpxchgXXb uses RBX as one of its implicit argument. I.e., when
we use that instruction we need to clobber RBX. This is generally
fine, expect when RBX is a reserved register because in that case,
the register allocator will not track its value and will not
save and restore it when interferences occur.

rdar://problem/24851412

llvm-svn: 262759