cmd/gc: replace x*8 by x<<3 etc

Fixes issue 4199.

R=ken2
CC=golang-dev
https://codereview.appspot.com/7322081
encoding/json: document and test use of unicode.ReplacementChar

Fixes issue 4783.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7314099
cmd/go: fix vet

The IgnoredGoFiles are already listed in allgofiles,
so they were being run twice. Worse, the ones in
IgnoredGoFiles are not fully qualified paths, so they
weren't being found when executed outside the
package directory.

Fixes issue 4764.

R=golang-dev, minux.ma, franciscossouza
CC=golang-dev
https://codereview.appspot.com/7308049