RFR: 8234248: More VectorSet cleanups

[Claes Redestad]

Hi,

JDK-8233708 removed unused code from VectorSet, but missed a few spots.

Additionally, the current code does a few other things like shifting up
signed longs and casting back to uint, which could be improved.

Bug:    https://bugs.openjdk.java.net/browse/JDK-8234248
Webrev: http://cr.openjdk.java.net/~redestad/8234248/open.00/

Testing: tier1-3

Thanks!

/Claes