This adds root-relative access in a couple of spots (e.g.: LoadRoot,
CompareRoot, PushRoot, Push, Move, etc.). Some methods now dispatch
based on whether the given Immediate is an embedded object.

ShouldGenerateIsolateIndependentCode() was added as a porting crutch:
it forces isolate-independent code for builtins even though the
builtin is not in the Builtins::IsIsolateIndependent() whitelist. This
allows us to easily figure out which builtins can be white-listed with
--print-embedded-builtin-candidates.

Newly isolate-independent builtins are now whitelisted.

Drive-by: Remove dead CompareRoot and JumpIfRoot helpers.

Bug: v8:6666
Change-Id: I6b5f6f71b90ac22e7e15482af0617b0ca1c319da
Reviewed-on: https://chromium-review.googlesource.com/c/1280665
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56673}