elide calls to empty glue

[graydon]

There are a few glue algorithms where we do structural recursion to generate
calls into sub-glue functions for each slot in a structured value
unnecessarily. We can discriminate between those structured values that have
sub-structure (or dtors) and those that are simple leaves, and omit calls to
their glue in some cases. For example, Trans.drop_slot on a structure like
@(int,int) will call a helper to 'drop' the interior bits of the exterior
tuple, despite the fact that it has no interesting substructure that needs
processing. We should notice this and avoid emitting the call (the called glue
is empty anyway).

[graydon]

WONTFIX (not required for bootstrapping, also LLVM does this automatically)