[turbofan] Optimize Splinter by remembering where it left off.

[turbofan] Optimize Splinter by remembering where it left off.

Splintering relies on DetachAt, which in turn relies on
FirstSearchIntervalForPosition to find the first UseInterval
to split, given a position. The later API (Find...) has an
optimization for linear traversals. Splintering traverses
linearly (block by block), so we leverage the same
optimization by moving current_interval_ forward.

(Also added an unrelated TODO.)

BUG=chromium:524880
LOG=n

Committed: https://crrev.com/1d9642d37fdbbd27d12c3ba99e96912447ce78e0
Cr-Commit-Position: refs/heads/master@{#30456}

[Mircea Trofin, 2015-08-28 21:49:27]

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

[Benedikt Meurer, 2015-08-31 06:01:20]

LGTM.

[Mircea Trofin, 2015-08-31 06:39:05]

The CQ bit was checked by mtrofin@chromium.org

[commit-bot: I haz the power, 2015-08-31 06:39:16]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1322623007/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1322623007/1

[commit-bot: I haz the power, 2015-08-31 07:06:13]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-08-31 07:06:37]

Patchset 1 (id:??) landed as
https://crrev.com/1d9642d37fdbbd27d12c3ba99e96912447ce78e0
Cr-Commit-Position: refs/heads/master@{#30456}