Similar to the feedback vector, we cannot embed the native context as
a constant in NCI code (it is trivially native-context-dependent). In
NCI mode, load it from the current context. In default turbofan, we
keep the HeapConstant.

Bug: v8:8888
Change-Id: Iff95c673b25245c701c7755416abf2038b5fdf08
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2282532
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68712}