RFR: 8270832: Aarch64: Update algorithm annotations for MacroAssembler::fill_words

[Wang Huang]

It is found that the comments of `MacroAssembler::fill_words` is not right here. Let's fix that.

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Commit messages:
 - 8270832: Aarch64: Update algorithm annotations for MacroAssembler::fill_words

Changes: https://git.openjdk.java.net/jdk/pull/4809/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4809&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8270832
  Stats: 14 lines in 1 file changed: 7 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4809.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4809/head:pull/4809

PR: https://git.openjdk.java.net/jdk/pull/4809