IsNumber returns true for Smi/HeapNumber arguments.

IsNumberNormalized expects a number argument and returns true if
it is either a Smi, or a HeapNumber with a value outside the Smi
range.

BUG=

Review-Url: https://codereview.chromium.org/2813623003
Cr-Commit-Position: refs/heads/master@{#44556}