The immediate passed to cmpw can be either a signed 16-bit or an
unsigned 16-bit integer, but the DCHECK was testing for signed 16-bit
values only.

R=mstarzinger@chromium.org
BUG=v8:6063

Review-Url: https://codereview.chromium.org/2735363002
Cr-Commit-Position: refs/heads/master@{#43671}