This simplifies the existing invariant and enables loop-peeling on all loops.
The main motivation is that it enables dead code elimination to always eagerly
fold away branches even when this would create infinite loops.

Bug: 
Change-Id: If4347f748f8d8735965771f66260a8f931b24132
Reviewed-on: https://chromium-review.googlesource.com/763531Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49387}