[cleanup] Unify A.p.includes and .indexOf CSA builtins

Change-Id: Iadd6764c3b478b4363c95fbfc9370fd8f47b6080
Reviewed-on: https://chromium-review.googlesource.com/505493
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45345}