[aarch64-port-dev ] RFR: 8222785: aarch64: add necessary masking for immediate shift counts

[Yangfei (Felix)]

Hi,

    Please review this patch adding necessary masking for immediate shift counts in aarch64.ad.
    Bug: https://bugs.openjdk.java.net/browse/JDK-8222785
    Webrev: http://cr.openjdk.java.net/~fyang/webrev.00/
    
    Previous discussion is here: https://mail.openjdk.java.net/pipermail/aarch64-port-dev/2019-April/007173.html
    As C2 does not mask immediate shifts, it's necessary to mask immediate shift counts in aarch64.ad. 
    Jtreg tested with an aarch64 fastdebug build. 

Thanks,
Felix