This operation was used to implement asm.js stores, but is obsolete
with asm.js stores now being lowered to normal graph nodes.

R=mstarzinger@chromium.org
CC=jarin@chromium.org

Bug: 
Change-Id: Iea90b1a62be2e273c0562058642adc5b63ae2cf8
Reviewed-on: https://chromium-review.googlesource.com/822570
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50046}