[turbofan] LiveRange splinter merging optimizations.

[turbofan] LiveRange splinter merging optimizations.

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:524880
LOG=n

Committed: https://crrev.com/1c6139de8bb4d26c2ce0278091388d5838311666
Cr-Commit-Position: refs/heads/master@{#30400}

[Mircea Trofin, 2015-08-27 00:53:22]

mtrofin@chromium.org changed reviewers:
+ bmeurer@chromium.org, jarin@chromium.org

[Benedikt Meurer, 2015-08-27 04:42:38]

lgtm

[Mircea Trofin, 2015-08-27 04:43:09]

The CQ bit was checked by mtrofin@chromium.org

[commit-bot: I haz the power, 2015-08-27 04:43:17]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1318893002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1318893002/1

[commit-bot: I haz the power, 2015-08-27 04:44:38]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-08-27 04:44:50]

Patchset 1 (id:??) landed as
https://crrev.com/1c6139de8bb4d26c2ce0278091388d5838311666
Cr-Commit-Position: refs/heads/master@{#30400}