Revert of [x64] Generate test reg,reg instead of cmp reg,0. (patchset #2 id:20001 of https://codereview.chromium.org/916543004/)

Reason for revert:
Tanks some benchmarks (for no obvious reason)

Original issue's description:
> [x64] Generate test reg,reg instead of cmp reg,0.
>
> The instruction sequence is shorter and saves decoding bandwidth.
>
> R=svenpanne@chromium.org
>
> Committed: https://crrev.com/a7fb5ed400701b22c455d0ad410bc98b211e5730
> Cr-Commit-Position: refs/heads/master@{#26734}

TBR=svenpanne@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/939923003

Cr-Commit-Position: refs/heads/master@{#26742}