GC bridge: Only export non-bridged objects when it's helpful

GC bridge: Only export non-bridged objects when it's helpful

Augments each tarjan-bridge SCC ("color") with a count of how many
SCCs are pointing to it. The mechanism for exporting non-bridged SCCs
uses this count to only export those SCCs where the "double fan"
scenario appears to be occurring. In testing this eliminates the
performance overhead imposed by the previous commit.

Also renamed/moved some variable declarations because namespace bugs
were creeping in.