fix & improve AST_TemplateString (#2410)

- resolve `semicolons:false` ambiguity with tagged literals
- allow `side_effects` to work on template literals
- traverse `AST_TemplateString` properly

fixes #2401

lib/compress.js

@@ -2230,6 +2230,10 @@ merge(Compressor.prototype, {
                 return expression.has_side_effects(compressor);
             });
         });
+        def(AST_TemplateSegment, return_false);
+        def(AST_TemplateString, function(compressor){
+            return any(this.segments, compressor);
+        });
     })(function(node, func){
         node.DEFMETHOD("has_side_effects", func);
     });
@@ -2944,6 +2948,11 @@ merge(Compressor.prototype, {
         def(AST_Expansion, function(compressor, first_in_statement){
             return this.expression.drop_side_effect_free(compressor, first_in_statement);
         });
+        def(AST_TemplateSegment, return_null);
+        def(AST_TemplateString, function(compressor){
+            var values = trim(this.segments, compressor, first_in_statement);
+            return values && make_sequence(this, values);
+        });
     })(function(node, func){
         node.DEFMETHOD("drop_side_effect_free", func);
     });
@@ -4888,7 +4897,7 @@ merge(Compressor.prototype, {
         }
         self.segments = segments;
 
-        return self;
+        return segments.length == 1 ? make_node(AST_String, self, segments[0]) : self;
     });
 
     OPT(AST_PrefixedTemplateString, function(self, compressor){