Rewrite test/Feature/md_on_instruction.ll

This test is supposed to be testing whether metadata attachments to
instructions work, but it was using invalid debug info to do so.  (This
was causing assertion failures in the `DebugInfoFinder` with a WIP patch
to be more strict about `DIDescriptor` accessors.)

Rather than fix the debug info -- which is better tested elsewhere --
just test the IR feature directly.

llvm-svn: 232828