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}
