This CL implements an assembly order optimization that moves blocks
that end a loop with an unconditional backedge to the beginning of
the loop, saving a branch.

R=jarin@chromium.org,mstarzinger@chromium.org
BUG=v8:8423

Change-Id: I8a5d25f5472d71227af0f623277ea8d0a8d69867
Reviewed-on: https://chromium-review.googlesource.com/c/1335944
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57572}