The bytecode interpreter nor TF rely on the explicit return value of store ICs anymore, so we can just return whatever is in the result slot. It won't be visible to JS anyway.

Bug: 
Change-Id: I389615d1d77c5b050832f23a08e3d3bc07d9cbc6
Reviewed-on: https://chromium-review.googlesource.com/743366Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49027}