RFR: 8279560: AArch64: generate_compare_long_string_same_encoding and LARGE_LOOP_PREFETCH alignment

[Wang Huang]

Fix on code shape :
```c++
      __ bind(LARGE_LOOP_PREFETCH);
        __ prfm(Address(str1, SoftwarePrefetchHintDistance));
        __ prfm(Address(str2, SoftwarePrefetchHintDistance));

        __ align(OptoLoopAlignment);

->

```c++
      __ align(OptoLoopAlignment);
      __ bind(LARGE_LOOP_PREFETCH);
        __ prfm(Address(str1, SoftwarePrefetchHintDistance));
        __ prfm(Address(str2, SoftwarePrefetchHintDistance));
``` 
Thanks to @shipilev  for your great advice. Any comments and suggestions will be appreciated.

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

Commit messages:
 - 8279560: AArch64: generate_compare_long_string_same_encoding and LARGE_LOOP_PREFETCH alignment

Changes: https://git.openjdk.java.net/jdk/pull/7007/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7007&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279560
  Stats: 2 lines in 1 file changed: 1 ins; 1 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7007.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7007/head:pull/7007

PR: https://git.openjdk.java.net/jdk/pull/7007