Using the proper `add` operation assures the NAN value
is properly propagated to the result register.

Change-Id: Icb86193f85534604f2a4a583d177a6f319ca38c3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2429804Reviewed-by: Junliang Yan <junyan@redhat.com>
Commit-Queue: Milad Farazmand <mfarazma@redhat.com>
Cr-Commit-Position: refs/heads/master@{#70127}