interchange: Don't hold reference to visit in global routing
interchange: Don't hold reference to visit in global routing

Signed-off-by: Krzysztof Boronski <17d64aa0ff0bac9be002185bb7702ff5dfb1cf8e@antmicro.com>

YosysHQ/nextpnr,YosysHQ/nextpnr,YosysHQ/nextpnr,YosysHQ/nextpnr