spec: narrow syntax for expression and select statements
spec: narrow syntax for expression and select statements

This is not a language change, it simply expresses the
accepted cases explicitly in the respective productions.

R=rsc, r, iant
CC=golang-dev
http://codereview.appspot.com/4428057

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang