- Only accept valid if statement syntax in go/parser.

- Check AST again in go/types since it may have been modified and the
  AST doesn't preclude other statements in the else branch of an if
  statement.

- Removed a test from gofmt which verified that old-style if statements
  permitting any statement in the else branch were correctly reformatted.
  It's been years since we switched to the current syntax; no need to
  support this anymore.

- Added a comment to go/printer.

Fixes #13475.

Change-Id: Id2c8fbcc68b719cd511027d0412a37266cceed6b
Reviewed-on: https://go-review.googlesource.com/17408Reviewed-by: Russ Cox <rsc@golang.org>