cmd/gc: racewalk: do not double function calls

cmd/gc: racewalk: do not double function calls
Current racewalk transformation looks like:
x := <-makeChan().c
\/\/\/\/\/\/\/\/\/
runtime.raceread(&makeChan().c)
x := <-makeChan().c
and so makeChan() is called twice.
With this CL the transformation looks like:
x := <-makeChan().c
\/\/\/\/\/\/\/\/\/
chan *tmp = &(makeChan().c)
raceread(&*tmp)
x := <-(*tmp)
Fixes #4245.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6822075