The type of StringIndexOf nodes is never recomputed since the operation
is simply changed on the original node.

Bug: chromium:769923
Change-Id: I3a2956ea69d43a56d22aff0607ac9869cf65533c
Reviewed-on: https://chromium-review.googlesource.com/718758Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48558}