Emit FIRRTL bulkconnects whenever possible (#2381)

Chisel <> semantics differ somewhat from FIRRTL <= semantics,
so we only emit <= when it would be legal. Otherwise we continue
the old behavior of emitting a connection for every leaf-level
Element.

Co-authored-by: Deborah Soung <debs@sifive.com>
Co-authored-by: Jack Koenig <koenig@sifive.com>