Almost safe optimization in AsmWriter
Could create problems only if AsmWriter is called before and after changes in a function.
optimization in AsmWriter that SHOULD be safe*, but I don't guarantee it.
Combined to the previous one, runtime of my pass drops from 6 minutes to 15 seconds
and the result is still correct, so I apply it anyway in my tree.

*As in previous commit, if AsmWriter is not called BEFORE AND AFTER modifications in a module.