go/printer: don't lose relevant parentheses when rewriting selector expressions
Also: Simplified handling of selector expressions. As a result, complicated
multi-line expressions containing selectors and calls/indices with arguments
broken accross lines don't get indented the same way as before, but the change
is minimal (see tests) and there's no such code in the std library. It seems
a worthwhile compromise given the much simpler code.
Applied gofmt -w $GOROOT/src $GOROOT/misc .
Fixes #1847.
R=rsc
CC=golang-dev
https://golang.org/cl/
5675062