[vm/compiler] Update outdated comment.
[vm/compiler] Update outdated comment.

Also outline a possible optimization here, with an corresponding
issue filed to later do that work.

Change-Id: I353cab8842cbbc0e2e15e127e45e736884d3c51e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/106343
Commit-Queue: Teagan Strickland <cce218d042444252b7ac446d60910fca7732559c@google.com>
Reviewed-by: Martin Kustermann <a35dd5b79fac84b2bb3b026a16b8becc55af5961@google.com>

dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk