kernel: look up ports when applying clock constraints. (YosysHQ#1448)

prjunnamed does not emit a net alias for toplevel ports. This works
fine for constraining IOs but breaks clock constraints. This commit
expands clock constraint application code to look up net aliases first,
ports second.

common/kernel/basectx.cc

@@ -100,12 +100,15 @@ void BaseCtx::addClock(IdString net, float freq)
     cc->period = DelayPair(getCtx()->getDelayFromNS(1000 / freq));
     cc->high = DelayPair(getCtx()->getDelayFromNS(500 / freq));
     cc->low = DelayPair(getCtx()->getDelayFromNS(500 / freq));
-    if (!net_aliases.count(net)) {
-        log_warning("net '%s' does not exist in design, ignoring clock constraint\n", net.c_str(this));
-    } else {
+    if (net_aliases.count(net)) {
         getNetByAlias(net)->clkconstr = std::move(cc);
-        log_info("constraining clock net '%s' to %.02f MHz\n", net.c_str(this), freq);
+    } else if (ports.count(net)) {
+        ports[net].net->clkconstr = std::move(cc);
+    } else {
+        log_warning("net '%s' does not exist in design, ignoring clock constraint\n", net.c_str(this));
+        return;
     }
+    log_info("constraining clock net '%s' to %.02f MHz\n", net.c_str(this), freq);
 }
 
 void BaseCtx::createRectangularRegion(IdString name, int x0, int y0, int x1, int y1)