Don't emit dead code branches that are guarded by boolean literals.
Add a single level of simplification for !true -> false, and
!false -> true.
While this might not be very common in code, it does allow for a
useful pattern in the runtime libraries. Branches can be included
into the compiled code or not based on a compile time flag.
Change-Id: Ib90e1e951cea3ef8c75b944635776b292759594a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/243363
Commit-Queue: Nicholas Shahan <nshahan@google.com>
Reviewed-by: Anna Gringauze <annagrin@google.com>
