Previously ChangeLowering would always box float64 values when going to
tagged representation, but that introduces a lot of deoptimizer loops
and polymorphism into TurboFan, which is unfortunate and unnecessary.

This adds some logic to ChangeFloat64ToTagged to try harder to create a
Smi when going from Float64 to Tagged, instead of always allocating a
HeapNumber.  This might need some additional tweaking, but at least it
makes it possible to start comparing TurboFan and Crankshaft for some
regular JavaScript.

R=jarin@chromium.org

Review URL: https://codereview.chromium.org/1420913003

Cr-Commit-Position: refs/heads/master@{#31609}