Speed up Speed up hb_set_digest_common_bits_t calcs

Correctly this time.

src/hb-set-private.hh

@@ -54,9 +54,11 @@ struct hb_set_digest_common_bits_t
   }
 
   inline void add_range (hb_codepoint_t a, hb_codepoint_t b) {
-    /* TODO Speedup. */
-    for (unsigned int i = a; i < b + 1; i++)
-      add (i);
+    add (a);
+    /* The negation here stands for ~(x-1). */
+    mask_t upper_bits = -(1 << _hb_bit_storage (a ^ b));
+    mask &= upper_bits;
+    value &= upper_bits;
   }
 
   inline bool may_have (hb_codepoint_t g) const {