RFR 8214239 (S): Missing x86_64.ad patterns for clearing and setting long vector bits

[B. Blaser]

Hi,

Please review the following fix for [1] which has been extensively
discussed in [2]:

http://cr.openjdk.java.net/~bsrbnd/jdk8214239/webrev.00/

It includes:
* Vladimir's requested changes about instruction encoding and testing
along with:
* John's suggested complementary benchmark following Sandhya's note
regarding throughput.

Thanks (hotspot:tier1 is OK on Linux/x86_64),
Bernard

[1] https://bugs.openjdk.java.net/browse/JDK-8214239
[2] http://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2019-November/035699.html