We eagerly inserted Int32Mul for Math.imul during builtin lowering and
messed up with the types, which confused the representation selection.
This adds a proper NumberImul operator, and fixes the builtin reducer to
do the right thing according to the spec.

R=mstarzinger@chromium.org
BUG=v8:5006
LOG=n

Review-Url: https://codereview.chromium.org/1971163002
Cr-Commit-Position: refs/heads/master@{#36219}