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}
