cmd/gc: fix double evaluation in interface comparison
cmd/gc: fix double evaluation in interface comparison

During interface compare, the operands will be evaluated twice. The operands might include function calls for conversion, so make them cheap before comparing them.

R=rsc
CC=golang-dev
http://codereview.appspot.com/6498133
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go