test/bench/garbage: fix parser benchmark

R=golang-dev, dvyukov
CC=golang-dev
https://codereview.appspot.com/7435053
cmd/gc: simplify and fix defaultlit.

Fixes issue 4882.
Fixes issue 4936.
Fixes issue 4937.

R=golang-dev, dave, daniel.morsing, rsc
CC=golang-dev
https://codereview.appspot.com/7432044