Verify 'optnone' can run DAG combiner when appropriate.

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

llvm-svn: 233584