While not strictly necessary, this is consistent with how
SlowFlagGetter behaves. It adds an additional shift operation (which
we could fold into the smi untagging if needed).

Drive-by: Typify flag accessors.

Bug: chromium:899464
Change-Id: Ib154d626e522ed723e2c19b1ab7f68560ac414bc
Reviewed-on: https://chromium-review.googlesource.com/c/1304315Reviewed-by: Peter Wong <peter.wm.wong@gmail.com>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57063}