frontend: don't connect a const net to ports connected to x.

prjunnamed normalizes ports that are not present in the primitive
to be all-x. On iCE40, this can cause a false placement conflict
between `SB_IO` cells where one's clock input is `x` and another's is
some other net.

frontend/frontend_base.h

@@ -472,7 +472,10 @@ template <typename FrontendType> struct GenericFrontend
                 ci->ports[port_bit_ids].type = dir;
                 // Resolve connectivity
                 NetInfo *net;
-                if (impl.is_vector_bit_constant(bits, i)) {
+                if (impl.is_vector_bit_undef(bits, i)) {
+                    // Don't connect it if it's an `x`
+                    continue;
+                } else if (impl.is_vector_bit_constant(bits, i)) {
                     // Create a constant driver if one is needed
                     net = create_constant_net(m, inst_name.str(ctx) + "." + port_bit_name + "$const",
                                               impl.get_vector_bit_constval(bits, i));

frontend/json_frontend.cc

@@ -161,6 +161,12 @@ struct JsonFrontendImpl
 
     int get_vector_length(BitVectorDataType &bits) const { return int(bits.size()); }
 
+    bool is_vector_bit_undef(BitVectorDataType &bits, int i) const
+    {
+        NPNR_ASSERT(i < int(bits.size()));
+        return bits[i] == "x";
+    }
+
     bool is_vector_bit_constant(BitVectorDataType &bits, int i) const
     {
         NPNR_ASSERT(i < int(bits.size()));