A few benchmarks, e.g. Massive/SQLite, turn out to be
sensitive to compile time. Upon analysis, splinter merging
and then splinter creation (splitting) appear to be the main
contributors to such regressions. This change tackles main
sources of regression in Merging. Profiling SQLite shows,
after this change, Merging as noise (down from main C++
contributor of samples)

BUG=chromium:1318893002
LOG=n

Review URL: https://codereview.chromium.org/1318893002

Cr-Commit-Position: refs/heads/master@{#30400}