[Debug info] Add new DbgValues after looping over DAG

[Debug info] Add new DbgValues after looping over DAG

I was contacted by Jesper Antonsson from Ericsson who ran into problems
with r311181 in their test suites with for an out-of-tree target.
Because of the latter I don't have a reproducer, but we definitely don't
want to modify the data structure on which we are iterating inside the
loop.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@311466 91177308-0d34-0410-b5e6-96231b3b80d8