RFR(S) 8031743: C2: loadI2L_immI broken for negative memory values

[Igor Veresov]

Transformation of (ConvI2L (AndI (LoadI mem) mask)) to (AndI (LoadUI2L (mem) mask) is only valid for positive mask, otherwise the sign extending effect of ConvI2L is missed. The solution is to restrict the optimization for the positive values of mask.

Webrev: http://cr.openjdk.java.net/~iveresov/8031743/webrev.00/
Testing: new regression test, jprt

Thanks!
igor