Revert "Revert of [turbofan] Splinter into one range.
(patchset #2 id:80001 of https://codereview.chromium.org/1391023007/ )"

This reverts commit 23a8837f.

Also added a CHECK in Merge to validate that splitting yields a different
range and thus advances the algorithm. Ran stress bots successfully. Likely my earlier change in Splintering addressed the stress test scenario
that was looping infinitely.

BUG=

Review URL: https://codereview.chromium.org/1406983004

Cr-Commit-Position: refs/heads/master@{#31430}