Adds a CheckElimination reducer to eliminate checks which have become
unecessary due to inlining of heap constants.

BUG=v8:6243, chromium:738312

Change-Id: Ie50b274bd07c86466eead08b2f21d2b63dd9e01c
Reviewed-on: https://chromium-review.googlesource.com/559129
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46460}