Use the simpler variant (that defaults to UndefinedConstant()) when
possible.

Bug: v8:6474
Change-Id: I46c6d5c2ffb98cb05f2db003d159dcd9113db085
Reviewed-on: https://chromium-review.googlesource.com/532977Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45904}