RFR(s): JDK-8214687 Optimize Collections.nCopies().hashCode()

[Tagir Valeev]

Hello!

Please review and sponsor the optimized implementation of
Collections.nCopies().hashCode():
https://bugs.openjdk.java.net/browse/JDK-8214687
http://cr.openjdk.java.net/~tvaleev/webrev/8214687/r1/

Previous discussion thread:
http://mail.openjdk.java.net/pipermail/core-libs-dev/2018-November/056843.html

Thanks to Zheka Kozlov for the original proposal. Also thanks to Ivan
Gerasimov for the simplification idea: (x & 0x8000_0000) != 0 => x <
0.

With best regards,
Tagir Valeev