X86: remove atomic instructions *after* we've iterated through them.

Otherwise they get freed and the implicit "isa<XYZ>" tests following
turn out badly (at least under sanitizers).

Also corrects the ordering of unordered atomic stores.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212136 91177308-0d34-0410-b5e6-96231b3b80d8