[vm, gc] Don't enqueue pre-marked Instructions.
[vm, gc] Don't enqueue pre-marked Instructions.

Change-Id: Ic9a3b5f83fc45e363162c0531d474a663d243c6c
Reviewed-on: https://dart-review.googlesource.com/c/92666
Reviewed-by: Régis Crelier <c2bf536a10d9f2530c38e4e8da517718be8c39ce@google.com>
Commit-Queue: Ryan Macnak <d738b7f001baf9789095d7dc549cff6de60b011d@google.com>

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