RFR(S): 8241475: AArch64: Add missing support for PopCountVI node

[Pengfei Li]

Hi,

Please help review this another missing node support for AArch64.

JBS: https://bugs.openjdk.java.net/browse/JDK-8241475
Webrev: http://cr.openjdk.java.net/~pli/rfr/8241475/webrev.01/

This adds missing support for C2 node "PopCountVI" for AArch64. This
node is now used on x86 and ppc for the codegen of vectorized bit count
of multiple lanes of ints.

In this AArch64 implementation, CNT instruction is used firstly to get
a per-byte bit count of the NEON register. Then two UADDLP instructions
are used to convert the per-byte counts to per-int counts.

[TESTS]
Jtreg: hotspot::hotspot_all_no_apps, jdk::jdk_core and langtools::tier1.
No new failure found. Intel has already contributed a jtreg case in the
x86 patch [1] for this.

JMH: Tested with a small JMH [2]
Before:
  Benchmark                  Mode  Cnt    Score    Error  Units
  TestPopCount.testPopCount  avgt   15  129.773 ± 36.120  us/op

After:
  Benchmark                  Mode  Cnt   Score   Error  Units
  TestPopCount.testPopCount  avgt   15  41.830 ± 0.068  us/op

Thanks Dmitrij for the suggestion on this patch.

[1] http://hg.openjdk.java.net/jdk/jdk/rev/b242a1e3f9cf
[2] http://cr.openjdk.java.net/~pli/rfr/8241475/TestPopCount.java

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Thanks,
Pengfei