This reverts commit f52b4b3b.

Reason for revert: Makes benchmarks time out:
https://crbug.com/834655

Original change's description:
> [compiler] Temporarily turn some DCHECKs into CHECKs.
> 
> This turns most DCHECKs in the register allocator code into CHECKs,
> which hopefully will tell us more about the crashes in crbug 831822.
> 
> This CL will eventually be reverted again.
> 
> Bug: chromium:831822
> Change-Id: I123ed507949ecab50bb0aaaf9e91978c9c2d8d65
> Reviewed-on: https://chromium-review.googlesource.com/1016762
> Commit-Queue: Georg Neis <neis@chromium.org>
> Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#52673}

TBR=neis@chromium.org,sigurds@chromium.org

Change-Id: Ic2b37a3ce04516c5871b801015153ce84e622e90
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:831822
Reviewed-on: https://chromium-review.googlesource.com/1018860Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52679}