Do not cprop through ternaries if the selector is an l-value.

It turns out that it is possible for an otherwise constant net
mux to be non-constant if there is a force that can drive the
net. This can be detected as an l-value reference to an otherwise
constant selector.
(cherry picked from commit abc4eb9d52)

cprop.cc

@@ -823,6 +823,10 @@ void cprop_functor::lpm_mux(Design*des, NetMux*obj)
 
 	/* If the select input is constant, then replace with a BUFZ */
       bool flag = obj->pin_Sel().nexus()->drivers_constant();
+	/* Note that this cannot be constant if there are assignments
+	   to this nexus. (Assignments include "force" to nets.) */
+      flag &= !obj->pin_Sel().nexus()->assign_lval();
+
       verinum::V sel_val = flag? obj->pin_Sel().nexus()->driven_value() : verinum::Vx;
       if ((sel_val != verinum::Vz) && (sel_val != verinum::Vx)) {
 	    NetBUFZ*tmp = new NetBUFZ(obj->scope(), obj->name(), obj->width());

net_link.cc

@@ -261,6 +261,25 @@ verinum::V Nexus::get_init() const
       return verinum::Vz;
 }
 
+bool Nexus::assign_lval() const
+{
+      assert(list_);
+      for (Link*cur = list_ ; cur ; cur = cur->next_) {
+
+	    const NetPins*obj;
+	    unsigned pin;
+	    cur->cur_link(obj, pin);
+	    const NetNet*net = dynamic_cast<const NetNet*> (obj);
+	    if (net == 0)
+		  continue;
+
+	    if (net->peek_lref())
+		  return true;
+      }
+
+      return false;
+}
+
 bool Nexus::drivers_present() const
 {
       assert(list_);

netlist.h

@@ -347,6 +347,11 @@ class Nexus {
 
       NetNet* pick_any_net();
 
+	/* This method returns true if there are any assignments that
+	   use this nexus as an l-value. This can be true if the nexus
+	   is a variable, but also if this is a net with a force. */
+      bool assign_lval() const;
+
 	/* This method returns true if there are any drivers
 	   (including variables) attached to this nexus. */
       bool drivers_present() const;