RFR: 8153713 : aarch64: improve short array clearing using store pair

[Felix Yang]

Hi,

    Please review webrev:
http://cr.openjdk.java.net/~fyang/8153713/webrev.00/
    JIRA Issue: https://bugs.openjdk.java.net/browse/JDK-8153713

    Currently, C2 compiler generate independent stores to clear short
arrays whose size is no bigger than parameter InitArrayShortSize (refer to
ClearArrayNode::Ideal function).  For the aarch64 port, we have store pair
instruction which can zero two memory words at a time and this will be good
for code size and maybe performance for some micro-archs.

    For Spark Terasort, an extra of 550 stp (xzr, xzr) instructions are
generated with the patch, which mean about 2KB reduction in codesize.

    Tested with JTreg hotspot, langtools and jdk.  Is it OK?

Thanks,
Felix
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