implement compress option reduce_funcs (#2466)

- inline single-use function declarations as expressions when permissible
- depend on `reduce_vars`
- enabled by default
- disable for speed critical code

fixes #2464

lib/compress.js

@@ -75,6 +75,7 @@ function Compressor(options, false_by_default) {
         properties    : !false_by_default,
         pure_getters  : !false_by_default && "strict",
         pure_funcs    : null,
+        reduce_funcs  : !false_by_default,
         reduce_vars   : !false_by_default,
         sequences     : !false_by_default,
         side_effects  : !false_by_default,
@@ -4255,6 +4256,7 @@ merge(Compressor.prototype, {
             var fixed = self.fixed_value();
             if (fixed instanceof AST_Defun) {
                 d.fixed = fixed = make_node(AST_Function, fixed, fixed);
+                if (!compressor.option("reduce_funcs")) d.single_use = false;
             }
             if (d.single_use && fixed instanceof AST_Function) {
                 if (d.escaped && d.scope !== self.scope || recursive_ref(compressor, d)) {