Split ToNumberStub into the entry ToNumberStub, and two new stubs,
StringToNumberStub and NonNumberToNumberStub, which can be used when we
already know something about the input (i.e. in various branches of the
code stubs, or in TurboFan graphs).

Also introduce an appropriate StringToNumber simplified operator for
TurboFan, that is pure and is lowered to an invocation of the newly
added StringToNumberStub.

R=jarin@chromium.org

Review URL: https://codereview.chromium.org/1818923002

Cr-Commit-Position: refs/heads/master@{#34922}