RFR: 8268229: Aarch64: Use Neon in intrinsics for String.equals

[Wang Huang]

Dear all, 
     Could you give me a favor to review this patch? It improves the performance of the intrinsic of `String.equals` on Neon backend of Aarch64.
     We profile the performance by using this JMH case:
 

   ```java
    package com.huawei.string;
    import java.util.*;
    import java.util.concurrent.TimeUnit;
    
    import org.openjdk.jmh.annotations.CompilerControl;
    import org.openjdk.jmh.annotations.Benchmark;
    import org.openjdk.jmh.annotations.Level;
    import org.openjdk.jmh.annotations.OutputTimeUnit;
    import org.openjdk.jmh.annotations.Param;
    import org.openjdk.jmh.annotations.Scope;
    import org.openjdk.jmh.annotations.Setup;
    import org.openjdk.jmh.annotations.State;
    import org.openjdk.jmh.annotations.Fork;
    import org.openjdk.jmh.infra.Blackhole;
    
    @State(Scope.Thread)
    @OutputTimeUnit(TimeUnit.MILLISECONDS)
    public class StringEqual {
        @Param({"8", "64", "4096"})
        int size;
    
        String str1;
        String str2;
    
        @Setup(Level.Trial)
        public void init() {
            str1 = newString(size, 'c', '1');
            str2 = newString(size, 'c', '2');
        }
    
        public String newString(int length, char charToFill, char lastChar) {
            if (length > 0) {
                char[] array = new char[length];
                Arrays.fill(array, charToFill);
                array[length - 1] = lastChar;
                return new String(array);
            }
            return "";
        }
    
        @Benchmark
        @CompilerControl(CompilerControl.Mode.DONT_INLINE)
        public boolean EqualString() {
            return str1.equals(str2);
        }
    }

   ```
The result is list as following:（Linux aarch64 with 128cores）

Benchmark                       | (size) |  Mode | Cnt  |     Score |     Error |  Units
----------------------------------|-------|---------|-------|------------|------------|----------
StringEqual.EqualString      |         8 | thrpt  | 10 | 123971.994 | ± 1462.131 | ops/ms
StringEqual.EqualString       |       64 | thrpt |  10  | 56009.960  | ±  999.734 | ops/ms
StringEqual.EqualString        |    4096 | thrpt |  10 |   1943.852 | ±  8.159 | ops/ms
StringEqual.EqualStringWithNEON    |   8 | thrpt |  10 | 120319.271  | ± 1392.185 | ops/ms
StringEqual.EqualStringWithNEON    |  64 | thrpt |  10 |  72914.767 | ± 1814.173 | ops/ms
StringEqual.EqualStringWithNEON  |  4096 | thrpt  | 10  |  2579.155 | ± 15.589 | ops/ms

Yours, 
WANG Huang

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

Commit messages:
 - 8268229: Aarch64: Use Neon in intrinsics for String.equals

Changes: https://git.openjdk.java.net/jdk/pull/4423/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4423&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8268229
  Stats: 28 lines in 2 files changed: 25 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4423.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4423/head:pull/4423

PR: https://git.openjdk.java.net/jdk/pull/4423