[turbofan] More efficient splintering.

[turbofan] More efficient splintering.

For live ranges with many use positions, such as those encountered in
some unity asm.js code, this change significantly reduces compile time
(e.g. benchmarks/Compile/slow_nbody1.js: from ~6s to 2s). The
improvement is solely due to regressions (fixed by this CL) due to
splintering.

This CL does not fully address compile time problems for large
functions in Turbofan, but constitutes a step in the right direction.

Committed: https://crrev.com/cbbfc0ff87caed730f7ba424b45563355219a14c
Cr-Commit-Position: refs/heads/master@{#31220}

[Mircea Trofin, 2015-10-12 15:23:03]

The CQ bit was checked by mtrofin@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-10-12 15:23:14]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1386253004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1386253004/1

[commit-bot: I haz the power, 2015-10-12 15:23:17]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-10-12 15:23:17]

Dry run: No L-G-T-M from a valid reviewer yet. Only full committers are
accepted.
Even if an L-G-T-M may have been provided, it was from a non-committer,
_not_ a full super star committer.
See http://www.chromium.org/getting-involved/become-a-committer
Note that this has nothing to do with OWNERS files.

[Mircea Trofin, 2015-10-12 15:30:01]

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

[Mircea Trofin, 2015-10-12 15:30:18]

Note that current benchmarks, while generally positively impacted,
do not highlight the same order of magnitude improvement. Perhaps
we should consider including some of the Compile benchmarks to our
usual rotation.

[Benedikt Meurer, 2015-10-12 17:48:42]

LGTM.

[Mircea Trofin, 2015-10-12 18:25:02]

The CQ bit was checked by mtrofin@chromium.org

[commit-bot: I haz the power, 2015-10-12 18:25:10]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1386253004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1386253004/1

[commit-bot: I haz the power, 2015-10-12 18:34:33]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-12 18:34:47]

Patchset 1 (id:??) landed as
https://crrev.com/cbbfc0ff87caed730f7ba424b45563355219a14c
Cr-Commit-Position: refs/heads/master@{#31220}