[arm] Fix recognition of VNEG.

TEST=mjsunit,unittests
R=svenpanne@chromium.org

Committed: https://chromium.googlesource.com/v8/v8/+/2aed882fe74dbf9210159babe30fedccd900e7cf

Review URL: https://codereview.chromium.org/762493006

Cr-Commit-Position: refs/heads/master@{#25515}