Fix RegexCompileFlags::Backwards having the same value as Optimize

That means every Optimized regex had the Backwards version
compiled as well, which doubled the time it took to compile them
and doubled the memory usage of regex.

This should improve #2152

src/regex_impl.hh

@@ -120,8 +120,8 @@ enum class RegexCompileFlags
     None     = 0,
     NoSubs   = 1 << 0,
     Optimize = 1 << 1,
-    Backward = 1 << 1,
-    NoForward = 1 << 2,
+    Backward = 1 << 2,
+    NoForward = 1 << 3,
 };
 constexpr bool with_bit_ops(Meta::Type<RegexCompileFlags>) { return true; }