Use the type of the RHS to compute a more precise output type.  In
particular, if the RHS is, say, 1, we now know that the result is a
subtype of Unsigned31.

R=jarin@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2706763003
Cr-Commit-Position: refs/heads/master@{#43340}