Elements on typed arrays are never looked up in the prototype chain, so
there's no point in depending on the prototype chain validity cells for
keyed stores to typed arrays. You just risk going megamorphic for
unrelated changes.

Bug: v8:6999
Change-Id: Id831de42a2c9eadfd5317ee9b5dbfaa207f236fe
Reviewed-on: https://chromium-review.googlesource.com/737789Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48898}