xchg with a mem operand has different locking semantics. If we unfold it
into a xchg r,r we will loose the implicit lock. Likewise we never want
to fold a register xchg into a memory one as it would be a lot slower.
This triggers during LLVM selfhost.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@304163 91177308-0d34-0410-b5e6-96231b3b80d8