Moves the decision whether to embed the constant or perform a lookup
through the builtins constants table to
CodeAssembler::UntypedHeapConstant.

Root constants continue to be embedded (and are later turned into
loads through root-register by the backend); non-root constants are
added to the constants table at generation-time and loaded from there
at runtime.

This allows us to remove the recently added boilerplate around
CallStub and CallRuntime in a follow-up.

Bug: v8:6666
Change-Id: Id981088e4b9d665c678acc9718383179f681f063
Reviewed-on: https://chromium-review.googlesource.com/931122
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51495}