I believe the code reads easier after this change. The original code
probably dates back to when we had 4 gap positions. Now that there
are only 2, the logic can be simpler by avoiding a loop and instead
treating each case explicitly: no gaps; gaps just at end; gaps at start and
maybe end. That way, it is also  easier to understand how the moves get
pushed downwards. This is what got me to make this change in the first
place: trying to work out a finer grained move optimization.

BUG=

Review URL: https://codereview.chromium.org/1543973002

Cr-Commit-Position: refs/heads/master@{#33016}