Catch exceptions by const-reference

Exceptions caught by value incur needless cost in C++, most of them can
be caught by const-reference, especially as nearly none are actually
used. This could allow compiler generate a slightly more efficient code.

src/tactic/bv/bv1_blaster_tactic.cpp

@@ -371,7 +371,7 @@ class bv1_blaster_tactic : public tactic {
                     for_each_expr_core<visitor, expr_fast_mark1, false, true>(proc, visited, f);
                 }
             }
-            catch (not_target) {
+            catch (const not_target &) {
                 return false;
             }
             return true;