When finding the initial element in A.p.reduce[Right], we did
exclude holes, but did not reflect this is the type, which still
included the hole. This CL inserts a TypeGuard to ensure that
Turbofan knows the initial element is never the hole.

Bug: chromium:804837
Change-Id: Ia118ddafb8e16dd5c02559fa23216c9b139dd59a
Reviewed-on: https://chromium-review.googlesource.com/880967
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50814}