moves, we move those to the node, and remove them from the
predecessors ("merge" them to the common node).

If only some of the moves are common, we don't do anything. This is
what this change addresses.

The bug linked below should be addressed by this change. The only
difference in codegen before/after the change that introduced the bug
was un-merged moves.

BUG=chromium:549262
LOG=N

Review URL: https://codereview.chromium.org/1527203002

Cr-Commit-Position: refs/heads/master@{#33481}