RFR(S): 8135069: C2 replaces range checks by unsigned comparison with -1

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/8135069/webrev.00/

Another corner case that’s not handled correctly by the code that folds 2 consecutive integer comparisons into a single unsigned comparison:

i < 0 || i > -1 is folded as i >u -1. It should be i >=u 0. I refactored the code so it’s easier to relate the comments and the logic.

Roland.