RFR: JDK-8196064: AArch64: Merging ld/st into ldp/stp in macro-assembler

[Zhongwei Yao]

Hi, all,

Bug:
https://bugs.openjdk.java.net/browse/JDK-8196064

Webrev:
http://cr.openjdk.java.net/~zyao/8196064/webrev.00

This patch merges adjacent load/store into ldp/stp in macro-assembler
as discussed in previous thread:
http://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2017-December/027957.html

My micro-benchmark case gives about 8% performance improvement with this patch.

And this patch is based on commit be48be2 due to recent build failure
on jdk/hs master, and it passes all jtreg tests.

Please help to review it.
-- 
Best regards,
Zhongwei