If the RHS is 0 and we have Smi feedback, speculate that the result (the LHS)
will continue to be in the Unsigned31 range.  This helps us avoid converting
the result to double when merging with Signed32.

R=jarin@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2709423002
Cr-Commit-Position: refs/heads/master@{#43415}