This will also be useful for JSCallReducer.

In order to avoid extra work, the CL restricts one path of the JSCall
lowering to functions from the own native context.

Bug: v8:7790
Change-Id: I9f3a478969d641da59661ff196fdedae8195d680
Reviewed-on: https://chromium-review.googlesource.com/c/1286335Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56733}