cmd/vet: eliminate false positives for slices in untagged literal test
Made possible by go/types, as long as the package type-checks OK.

Fixes issue 4684.

R=golang-dev
CC=golang-dev
https://codereview.appspot.com/7407045
cmd/vet: fix up some nits in print.go found by kamil.kisiel@gmail.com

R=golang-dev, kamil.kisiel, bradfitz
CC=golang-dev
https://codereview.appspot.com/7369049