[15] RFR(S): 8236721: C2 should better optimize not-equal integer comparisons

[Tobias Hartmann]

Hi,

please review the following patch:
https://bugs.openjdk.java.net/browse/JDK-8236721
http://cr.openjdk.java.net/~thartmann/8236721/webrev.00/

While working on JDK-8229855 [1], I've noticed that C2 is not able to fold not-equal integer
comparisons of the following kind:

val = int:lo..hi
if (val != lo) {
  if (val <= lo) {
    // Dead code
  }
}

In general, C2's type system is quite limited when it comes to representing "sparse" types. But if
an integer value is found to be unequal to it's lower/upper boundary, we can still narrow the type.

With this enhancement, all checks in the 8 methods in TestFoldNECompares.java are folded and the
methods are compiled as empty.

Thanks,
Tobias

[1] https://bugs.openjdk.java.net/browse/JDK-8229855