While we may have consecutive deferred blocks, they may be entered
from other blocks that are not predecessors of the first block. Same
for the last block and exiting. So the total set of live ranges within
the given set of blocks is the union of the individual in and out live
sets.

This is a performance bug, not a functional bug.

BUG=

Review URL: https://codereview.chromium.org/1322703003

Cr-Commit-Position: refs/heads/master@{#30564}