This essentially performs the following transformation

  JSToNumber(phi(x1,...,xn,control):primitive)
    => phi(JSToNumber(x1),...,JSToNumber(xn),control):number

which is similar to what we already do for JSToBoolean.

TEST=mjsunit/asm
R=svenpanne@chromium.org
BUG=

Review URL: https://codereview.chromium.org/732463003

Cr-Commit-Position: refs/heads/master@{#25390}