RFR: 8270832: Aarch64: Update algorithm annotations for MacroAssembler::fill_words [v2]

[Wu Yan]

On Tue, 20 Jul 2021 06:36:10 GMT, Wang Huang <whuang at openjdk.org> wrote:

>> It is found that the comments of `MacroAssembler::fill_words` is not right here. Let's fix that.
>
> Wang Huang has updated the pull request incrementally with one additional commit since the last revision:
> 
>   fix comments

Could you do me a favor to review the patch? @theRealAph @nick-arm Thank you.

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