After https://crrev.com/c/832457, CheckedLoad is not being used any
more. Thus, remove it from the TurboFan backend and from all tests.
CheckedStore was already removed in https://crrev.com/c/822570.

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

Change-Id: I2eeec2f4a9d0a10067db5cc25ec41366ae85e917
Reviewed-on: https://chromium-review.googlesource.com/832459
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50323}