Add effect input and output to String.p.char[Code]At/codePointAt.
This is necessary to fix an hard to reproduce bug, a repro for
which is included. However, the only way to get the repro
included in this CL to fail is to run it with the patch of

  873382:
  [turbofan] Speculate on bounds checks for String#char[Code]At

but WITHOUT this patch. This fixes a scheduling problem triggered
by 873382 that caused a bounds check to get scheduled after the
associated access.

Bug: v8:7326
Change-Id: I4b97c1726caac92ff8f74c23df2788f0ecfb1304
Reviewed-on: https://chromium-review.googlesource.com/881781Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50832}