ecp5: Fix handling of missing ports in LUT permutation

Fixes #310

Signed-off-by: David Shah <dave@ds0.me>

ecp5/arch_place.cc

diff --git a/ecp5/arch_place.cc b/ecp5/arch_place.cc
index 18374c0..d5c345a 100644
--- a/ecp5/arch_place.cc
+++ b/ecp5/arch_place.cc
@@ -140,6 +140,10 @@
         std::vector<NetInfo *> orig_nets;
 
         for (int i = 0; i < 4; i++) {
+            if (!ci->ports.count(port_names.at(i))) {
+                ci->ports[port_names.at(i)].name = port_names.at(i);
+                ci->ports[port_names.at(i)].type = PORT_IN;
+            }
             auto &port = ci->ports.at(port_names.at(i));
             float crit = 0;
             if (port.net != nullptr && nc.count(port.net->name)) {