added option for side-effect-free statements, fix test

lib/compress.js

@@ -64,6 +64,7 @@ function Compressor(options, false_by_default) {
         if_return     : !false_by_default,
         join_vars     : !false_by_default,
         cascade       : !false_by_default,
+        side_effects  : !false_by_default,
 
         warnings      : true,
         global_defs   : {}
@@ -998,10 +999,12 @@ merge(Compressor.prototype, {
     });
 
     OPT(AST_SimpleStatement, function(self, compressor){
+        if (compressor.option("side_effects")) {
             if (!self.body.has_side_effects()) {
                 compressor.warn("Dropping side-effect-free statement [{file}:{line},{col}]", self.start);
                 return make_node(AST_EmptyStatement, self);
             }
+        }
         return self;
     });
 
@@ -1153,7 +1156,7 @@ merge(Compressor.prototype, {
                 operator: "&&",
                 left: self.condition,
                 right: self.body.condition
-            });
+            }).transform(compressor);
             self.body = self.body.body;
         }
         if (aborts(self.body)) {

test/compress/conditionals.js

@@ -124,3 +124,20 @@ ifs_5: {
         }
     }
 }
+
+ifs_6: {
+    options = {
+        conditionals: true,
+        comparisons: true
+    };
+    input: {
+        if (!foo && !bar && !baz && !boo) {
+            x = 10;
+        } else {
+            x = 20;
+        }
+    }
+    expect: {
+        x = foo || bar || baz || boo ? 20 : 10;
+    }
+}