Add a CanonicalizationMode to CodeStubAssembler::ChangeFloat64ToTagged,
so clients can request Smi canonicalization when desired, but otherwise
get Crankshaft/Fullcodegen compatible behavior of just boxing the double
into a HeapNumber.

R=verwaest@chromium.org
BUG=v8:5268

Review-Url: https://codereview.chromium.org/2380543002
Cr-Commit-Position: refs/heads/master@{#39804}