RFR: 8342601: AArch64: Micro-optimize bit shift in copy_memory

[Chad Rakoczy]

[JDK-8342601](https://bugs.openjdk.org/browse/JDK-8342601)

Fix minor inefficiency in `copy_memory` by adding check before doing bit shift to see if we are able to do a move instruction instead. Change is low risk because of the low complexity of the change

Ran array copy and tier 1on aarch64 machine

Test summary
==============================
   TEST                                              TOTAL  PASS  FAIL ERROR   
   jtreg:test/hotspot/jtreg/compiler/arraycopy          49    49     0     0   
==============================

==============================
Test summary
==============================
   TEST                                              TOTAL  PASS  FAIL ERROR   
   jtreg:test/hotspot/jtreg:tier1                     2591  2591     0     0   
   jtreg:test/jdk:tier1                               2436  2436     0     0   
   jtreg:test/langtools:tier1                         4577  4577     0     0   
   jtreg:test/jaxp:tier1                                 0     0     0     0   
   jtreg:test/lib-test:tier1                            34    34     0     0   
==============================

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/21589/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21589&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342601
  Stats: 6 lines in 1 file changed: 5 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/21589.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21589/head:pull/21589

PR: https://git.openjdk.org/jdk/pull/21589