Filter out remembered slots that are at the start of an object.

Filter out remembered slots that are at the start of an object.

These slots are invalid and can result in a broken offset when slot index and start of object are equal and are at the beginning of a cell.

Moreover, make DCHECKs CHECKs to catch bugs in the wild.

BUG=chromium:473174
LOG=n

Review URL: https://codereview.chromium.org/1051243004

Cr-Commit-Position: refs/heads/master@{#27602}