cmd/gc: silence valgrind error

valgrind complained that under some circumstances,

    *nr = *nc

was being called when nr and nc were the same *Node. The suggestion my R?my was to introduce a tmp node to avoid the potential for aliasing in subnode.

R=remyoudompheng, minux.ma, rsc
CC=golang-dev
https://codereview.appspot.com/7780044