gofmt: stability improvement

gofmt: stability improvement

There are a variety of token pairs that if printed
without separating blank may combine into a different
token sequence. Most of these (except for INT + .)
don't happen at the moment due to the spacing
introduced between expression operands. However, this
will prevent errors should the expression spacing
change.

R=rsc
CC=golang-dev
https://golang.org/cl/2207044