This removes LoadStackPointer and its last remaining use in the
interpreter assembler.

Bug: v8:9534
Change-Id: I19aafb12c5fd50248841a3d92448e64243c723ad
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1748729
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63164}