This was initially used to special case some weird date.js behavior, but
has since been abused in other areas. In case of the string character
access, everything that is outside the Smi range cannot be a valid
string index anyways, so %NumberToSmi is perfect here in either case,
and for ToPositiveInteger it's better to just use ToInteger adding +0 to
turn -0 into +0.

R=ishell@chromium.org
BUG=v8:5049

Review-Url: https://codereview.chromium.org/2010183003
Cr-Commit-Position: refs/heads/master@{#36545}