Not recording the address being taken was causing
the liveness analysis not to preserve x in the absence
of direct references to x, which in turn was making the
net test fail with GOGC=0.

In addition to the test, this fixes a bug wherein
        GOGC=0 go test -short net
crashed if liveness analysis was in use (like at tip, not like Go 1.2).

TBR=ken2
CC=golang-codereviews
https://golang.org/cl/64470043