RFR: Replace 'HEY!' comments with TODO and remove unnecessary usages

[Kelvin Nilsen]

On Tue, 12 Oct 2021 16:48:55 GMT, William Kemper <wkemper at openjdk.org> wrote:

> Replace idiosyncratic `HEY!` comments with `TODO` and remove it from places where it is no longer necessary. No code changes here, just changes in comments.

Marked as reviewed by kdnilsen (Committer).

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PR: https://git.openjdk.java.net/shenandoah/pull/87