part of a compile time improvement push. We got from 3 minutes down
to ~30 seconds prior to the change here.

This change further reduces the compile time down to 2 seconds, which
is actually slightly better than the pre-splintering total execution time
of about 3 seconds.

The cause of the regression was the repeated traversal of the children
of a live range, seeking for the one covering a safe point. The fix is to
leverage the intrinsic ordering in the chain of live range children, as well
as that of the safe points.

BUG= chromium:567745
LOG=N

Review URL: https://codereview.chromium.org/1529293002

Cr-Commit-Position: refs/heads/master@{#32958}