RFR(S): 8252696: Loop unswitching may cause out of bound array load to be executed

[Roland Westrelin]

I see Christian is out for a couple weeks. I added the verification code
that I had removed back as it's not directly related to the bug fix and
can be discussed further when Christian is back. I also made the small
changes he suggested:

http://cr.openjdk.java.net/~roland/8252696/webrev.01/

Roland.