TurboFan truncated null to +0 even in contexts such as -0 == null
because it was not handling the TypeCheck correctly. This restricts
the type conversion case to not apply truncation in this case (see
comment in patch).

Change-Id: Ia38ace9608800c8d61988de402a31dd863d9160a
Bug: chromium:961237
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1609538Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61446}