Up to now, for ranges spilled in deferred blocks, we would spill every
time a range would switch from using a register to spill slots. That can
be redundant, leading to avoidable code size  cost.

This change addresses this issue, by performing the spills as early as
possible.

BUG=

Review URL: https://codereview.chromium.org/1551013002

Cr-Commit-Position: refs/heads/master@{#33413}