[turbofan] Turn DCHECK for fixed slot index into a CHECK.

This is a temporary workaround to ensure that we crash in release mode
instead of running into undefined behavior.

BUG=chromium:444681
LOG=y
R=ishell@chromium.org

Review URL: https://codereview.chromium.org/800713006

Cr-Commit-Position: refs/heads/master@{#25933}