This splits the work to be done for dead and life maps
and avoids unnecessary operations in the hot loop.

Results for v8.infinite_scroll:

Before:
nonlive_refs
  len: 93
  min: 0.0
  max: 6.3
  avg: 1.35268817204
  [0,5[: 91
  [5,10[: 2

After:
nonlive_refs
  len: 91
  min: 0.0
  max: 4.2
  avg: 0.968131868132
  [0,5[: 91

BUG=chromium:554488
LOG=no

Review URL: https://codereview.chromium.org/1441633002

Cr-Commit-Position: refs/heads/master@{#32042}