DI: Update tests before adding !dbg subprogram attachments

I'm working on adding !dbg attachments to functions (PR23367), which
we'll use to determine the canonical subprogram for a function (instead
of the `subprograms:` array in the compile units).  This updates a few
old tests in preparation.

Transforms/Mem2Reg/ConvertDebugInfo2.ll had an old-style grep+count
based test that would start to fail because I've added an extra line
with `!dbg`.  Instead, explicitly `CHECK` for what I think the test
actually cares about.

All three testcases have subprograms with a valid `function:` reference
-- which means my upgrade script will add a `!dbg` attachment -- but
that aren't referenced from any compile unit.  I suspect these testcases
were handreduced over-zealously (or have bitrotted?).  Add a reference
from the compile unit so that upcoming Verifier checks won't fail here.

llvm-svn: 246351