go/ast, parser: condition in if statement is mandatory

go/ast, parser: condition in if statement is mandatory

As a result, parsing a "control clause" is now sufficiently
different for if, switch, and for statements that the code
is not factored out anymore. The code is a bit longer but
clearer in each individual case.

Reflect the changes in AST.

R=r, r2
CC=golang-dev
https://golang.org/cl/4173075