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

Review URL: https://codereview.chromium.org/1322623007

Cr-Commit-Position: refs/heads/master@{#30456}