grt: don't insert into routes_ when you only want to lookup
grt: don't insert into routes_ when you only want to lookup

Change various uses of routes_[net] to routes_.find(net).  Also
check if a net has actually been routed.

This is a partial fix for #1809 but the problem then moves into order wires.

Signed-off-by: Matt Liberty <44ef1ce0ef6935daf739a0795dad249ba5322e97@eng.ucsd.edu>

QuantamHD/OpenROAD,QuantamHD/OpenROAD,The-OpenROAD-Project/OpenROAD,The-OpenROAD-Project/OpenROAD,QuantamHD/OpenROAD,The-OpenROAD-Project/OpenROAD,The-OpenROAD-Project/OpenROAD,The-OpenROAD-Project/OpenROAD,QuantamHD/OpenROAD,QuantamHD/OpenROAD