RFR (S) 8150180: String.value contents should be trusted

[Vladimir Ivanov]

>    http://cr.openjdk.java.net/~shade/8150180/webrev.hs.01/

Why don't you check that index is also a constant? Otherwise, C2 can't 
constant fold the loads.

+  if (value->is_Con()) {
+    return false;
+  }

Best regards,
Vladimir Ivanov