The Crankshaft fast case for String.fromCharCode() unconditionally
deoptimizes on all non-int32 inputs, even tho it would be perfectly
valid to just truncate the index to an int32.

R=ishell@chromium.org
BUG=chromium:587068
LOG=n

Review URL: https://codereview.chromium.org/1727873003

Cr-Commit-Position: refs/heads/master@{#34243}