RFR: 8269559: AArch64: Implement string_compare intrinsic in SVE

[TatWai Chong]

This patch implements string_compare intrinsic in SVE.
It supports all LL, LU, UL and UU comparisons.

As we haven't found an existing benchmark to measure performance impact,
we created a benchmark derived from the test [1] for this evaluation.
This benchmark is attached to this patch.

Besides, remove the unused temporary register `vtmp3` from the existing
match rules for StrCmp.

The result below shows all varients can be benefited largely.
Command: make exploded-test TEST="micro:StringCompareToDifferentLength"

Benchmark    (size)  Mode  Cnt   Score Speedup Units
compareToLL      24  avgt   10      1.0x       ms/op
compareToLL      36  avgt   10      1.0x       ms/op
compareToLL      72  avgt   10      1.0x       ms/op
compareToLL     128  avgt   10      1.4x       ms/op
compareToLL     256  avgt   10      1.8x       ms/op
compareToLL     512  avgt   10      2.7x       ms/op
compareToLU      24  avgt   10      1.6x       ms/op
compareToLU      36  avgt   10      1.8x       ms/op
compareToLU      72  avgt   10      2.3x       ms/op
compareToLU     128  avgt   10      3.8x       ms/op
compareToLU     256  avgt   10      4.7x       ms/op
compareToLU     512  avgt   10      6.3x       ms/op
compareToUL      24  avgt   10      1.6x       ms/op
compareToUL      36  avgt   10      1.7x       ms/op
compareToUL      72  avgt   10      2.2x       ms/op
compareToUL     128  avgt   10      3.3x       ms/op
compareToUL     256  avgt   10      4.4x       ms/op
compareToUL     512  avgt   10      6.1x       ms/op
compareToUU      24  avgt   10      1.0x       ms/op
compareToUU      36  avgt   10      1.0x       ms/op
compareToUU      72  avgt   10      1.4x       ms/op
compareToUU     128  avgt   10      2.2x       ms/op
compareToUU     256  avgt   10      2.6x       ms/op
compareToUU     512  avgt   10      3.7x       ms/op

[1] https://github.com/openjdk/jdk/blob/master/test/hotspot/jtreg/compiler/intrinsics/string/TestStringCompareToDifferentLength.java

-------------

Commit messages:
 - 8269559: AArch64: Implement string_compare intrinsic in SVE

Changes: https://git.openjdk.java.net/jdk/pull/5129/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5129&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269559
  Stats: 550 lines in 12 files changed: 447 ins; 12 del; 91 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5129.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5129/head:pull/5129

PR: https://git.openjdk.java.net/jdk/pull/5129