go/parser, go/types: report invalid else branch in if statements
- 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.