[X86] Fix latent bugs in 32-bit CMPXCHG8B inserter

I found three issues:
1. the loop over E[ABCD]X copies run over BB start
2. the direct address of cmpxchg8b could be a frame index
3. the displacement of cmpxchg8b could be a global instead of an
   immediate

These were all introduced together in r287875, and should be fixed with
this change.

Issue reported by Zachary Turner.

llvm-svn: 371678