X86: remove temporary atomicrmw used during lowering.

We construct a temporary "atomicrmw xchg" instruction when lowering atomic
stores for widths that aren't supported natively. This isn't on the top-level
worklist though, so it won't be removed automatically and we have to do it
ourselves once that itself has been lowered.

Thanks Saleem for pointing this out!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212948 91177308-0d34-0410-b5e6-96231b3b80d8