[vector] Fix variable shift count amount

[Lupusoru, Razvan A]

Hi everyone,

This is a fix to follow up on an issue that Paul's comments led to its discovery. Namely that scalar implementation of shift implicitly truncates the shift count while the vector instructions actually saturate on the shift count. This patch fixes this issue. We will be working on adding a few tests soon so stay posted - they will be done as a follow up change.
http://cr.openjdk.java.net/~rlupusoru/panama/webrev_varshiftcountfix_00/index.html

Thanks,
Razvan