This removes a special case from JSObject::WriteToField() where we
didn't store anything in case of initializing a double field with
the uninitialized sentinel. Instead we now store the hole NaN pattern
there, as in other places. This makes it possible to do stricter
checking in the TurboFan frontend when it comes to detecting bit
patterns.

Drive-by-fix: Refactor the related code in MigrateFastToFast() to
make it easier to follow the control flow.

Bug: v8:9299
Change-Id: Ic35d05c69fbbb136d422d29ce6abf2b09ebe22a6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1631606Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Auto-Submit: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61879}