[turbofan] Greedy: smarter last resort splitting.

[turbofan] Greedy: smarter last resort splitting.

When all heuristics fail, we run a "last resort" heuristic. Before, it was
splitting at the first found splittable position either before or after a
use position. That turns out to be too naive: it may split in loops, when
alternative split positions exist outside loops.

This change chooses a "before" use case location that is outside the
loop.

Review URL: https://codereview.chromium.org/1372213005

Cr-Commit-Position: refs/heads/master@{#30990}