Fix regexp bug reported by Ian where [6-9] would match any digit.

src/jsregexp.cc

Index: src/jsregexp.cc
===================================================================
--- src/jsregexp.cc	(revision 2224)
+++ src/jsregexp.cc	(working copy)
@@ -1885,7 +1885,8 @@
         uint32_t differing_bits = (from ^ to);
         // A mask and compare is only perfect if the differing bits form a
         // number like 00011111 with one single block of trailing 1s.
-        if ((differing_bits & (differing_bits + 1)) == 0) {
+        if ((differing_bits & (differing_bits + 1)) == 0 &&
+             from + differing_bits == to) {
           pos->determines_perfectly = true;
         }
         uint32_t common_bits = ~SmearBitsRight(differing_bits);