When the incoming value to ChangeFloat64ToTagged is in Smi range, we
represent it as Smi instead of a HeapNumber. This addresses a range of
problems where TurboFan unnecessarily deoptimizes because an operation
learned Smi feedback in Ignition, but was then confronted with a tagged
HeapNumber in TurboFan, just because the value was also represented as
unboxed double somewhere in the meantime.

BUG=v8:6256
R=yangguo@chromium.org

Review-Url: https://codereview.chromium.org/2815283002
Cr-Commit-Position: refs/heads/master@{#44631}