Zeroing unused registers is not required. Removing it makes the code
very slightly smaller and very slightly faster.

Change-Id: I1ec17b497db971ca8a3641e3e94c063571419f27
GitHub-Last-Rev: f721bb263637717e8ff9fd2c34148b5b2762e8c4
GitHub-Pull-Request: golang/go#31596
Reviewed-on: https://go-review.googlesource.com/c/go/+/173160Reviewed-by: Ian Lance Taylor <iant@golang.org>