introduce unsafe_regexp (#1970)

fixes #1964

README.md

@@ -353,6 +353,9 @@ to set `true`; it's effectively a shortcut for `foo=true`).
 - `unsafe_proto` (default: false) -- optimize expressions like
   `Array.prototype.slice.call(a)` into `[].slice.call(a)`
 
+- `unsafe_regexp` (default: false) -- enable substitutions of variables with
+  `RegExp` values the same way as if they are constants.
+
 - `conditionals` -- apply optimizations for `if`-s and conditional
   expressions
 
@@ -443,10 +446,10 @@ to set `true`; it's effectively a shortcut for `foo=true`).
 - `keep_infinity` -- default `false`. Pass `true` to prevent `Infinity` from
   being compressed into `1/0`, which may cause performance issues on Chrome.
 
-- `side_effects` -- default `false`. Pass `true` to potentially drop functions
-marked as "pure".  A function call is marked as "pure" if a comment annotation
-`/*@__PURE__*/` or `/*#__PURE__*/` immediately precedes the call. For example:
-`/*@__PURE__*/foo()`;
+- `side_effects` -- default `true`. Pass `false` to disable potentially dropping
+  functions marked as "pure".  A function call is marked as "pure" if a comment
+  annotation `/*@__PURE__*/` or `/*#__PURE__*/` immediately precedes the call. For
+  example: `/*@__PURE__*/foo();`
 
 
 ### The `unsafe` option