Similar to http://crrev.com/2410883003 we don't need to do a minus zero
check for the right hand side of CheckedInt32Add, because we already
know that the left hand side cannot be minus zero, and the only way that
addition can yield -0 is (-0) + (-0).

R=jarin@chromium.org

Review URL: https://codereview.chromium.org/2431233003 .

Cr-Commit-Position: refs/heads/master@{#40421}