We completely un-wired the greedy allocator to focus on the
stackchecks in loops (splintering) work. This change re-wires greedy,
still behind its flag. For now, enabling the greedy allocator disables
the stackchecks in loops feature (and range splintering), so that we are
at the baseline we left it at.

The main contribution in this change is adapting the codebase after
the live range model refactoring, whereby RegisterAllocationData's
live_ranges() contains just top-level ranges, and children are accessed
via their parents.

BUG=

Review URL: https://codereview.chromium.org/1320363002

Cr-Commit-Position: refs/heads/master@{#30492}