6g: delete unnecessary OXXX initialization.
6g: delete unnecessary OXXX initialization.

No longer necessary after https://codereview.appspot.com/6497073/
removed the `if(n5.op != OXXX) { regfree(&n5); }`.

R=remy, r
CC=golang-dev, rsc
http://codereview.appspot.com/6498101
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go