drop lone "use strict" in function body (#2963)

fixes #2961

lib/compress.js

@@ -3024,6 +3024,16 @@ merge(Compressor.prototype, {
         return self;
     });
 
+    OPT(AST_Lambda, function(self, compressor){
+        tighten_body(self.body, compressor);
+        if (compressor.option("side_effects")
+            && self.body.length == 1
+            && self.body[0] === compressor.has_directive("use strict")) {
+            self.body.length = 0;
+        }
+        return self;
+    });
+
     AST_Scope.DEFMETHOD("drop_unused", function(compressor){
         if (!compressor.option("unused")) return;
         if (compressor.has_directive("use asm")) return;

test/compress/functions.js

@@ -2022,3 +2022,32 @@ deduplicate_parenthesis: {
     }
     expect_exact: "({}).a=b;({}.a=b)();(function(){}).a=b;(function(){}.a=b)();"
 }
+
+drop_lone_use_strict: {
+    options = {
+        side_effects: true,
+    }
+    input: {
+        function f1() {
+            "use strict";
+        }
+        function f2() {
+            "use strict";
+            function f3() {
+                "use strict";
+            }
+        }
+        (function f4() {
+            "use strict";
+        })();
+    }
+    expect: {
+        function f1() {
+        }
+        function f2() {
+            "use strict";
+            function f3() {
+            }
+        }
+    }
+}