Make sure only one copy of a filter is placed in the

Make sure only one copy of a filter is placed in the
exception handling table if we encounter it multiple
times.  Filters could be folded harder than this, but
that would mean a lot more work for not much gain.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@37908 91177308-0d34-0410-b5e6-96231b3b80d8