[vm/concurrency] Fix Function bit field setters.
[vm/concurrency] Fix Function bit field setters.

This is follow-up to 182d0dd156e58196a5e67555a5158524555d1bac.

TEST=flaky vm/dart_2/isolates/reload_many_isolates_live_and_die_test

Fixes https://github.com/dart-lang/sdk/issues/45169.

Change-Id: Ie59d26b7fc0594940742b37fe9ed0127e5871245
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/190040
Reviewed-by: Martin Kustermann <a35dd5b79fac84b2bb3b026a16b8becc55af5961@google.com>
Commit-Queue: Alexander Aprelev <dfb74c35a357c2059971a4bf7fe26b6a7c18085d@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk