Handle decoration groups with no decorations. (#1921)

In DecorationManager::RemoveDecorationsFrom, we do not remove the id
from a decoration group if the group has no decorations.  This causes
problems because KillNamesAndDecorates is suppose to remove all
references to the id, but in this case, there is still a reference.

This is fixed by adding a special case.

Also, there is the possibility of a double free because
RemoveDecorationsFrom will delete the instructions defining |id| when
|id| is a decoration group.  Later, KillInst would later write to memory
that has been deleted when trying to turn it into a Nop.  To fix this,
we will only remove the decorations that use |id| and not its definition
in RemoveDecorationsFrom.