Related to 1318893002 - another source of regressions in
benchmarks sensitive to compile time is the splintering
logic. This change addresses some, but not all, of that. In
particular, there are still some places (figuring out if a
range has a hole right where a deferred set of blocks is)
that need another look.

BUG=chromium:1318893002
LOG=n

Review URL: https://codereview.chromium.org/1319843002

Cr-Commit-Position: refs/heads/master@{#30425}