Adds a test to verify the behavior that r233153 restored: 'optnone'
does not spuriously disable the DAG combiner, and in fact there are
cases where the DAG combiner must run (even at -O0 or 'optnone') in
order for codegen to succeed.
Differential Revision: http://reviews.llvm.org/D8614
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@233584 91177308-0d34-0410-b5e6-96231b3b80d8
