cmd/vet: check for useless assignments to struct fields

[josharian]

Implement this TODO from the assignment checker:

// TODO: should also check for assignments to struct fields inside methods
// that are on T instead of *T. 

Discussion at https://groups.google.com/forum/#!topic/golang-nuts/rSTE4G75QQU/discussion.

This might also be extended to range loops -- see http://play.golang.org/p/FMsMT0dnhf,
linked from issue #8356.