Don't print ()'s around a range clause's expression.

Fixes #605.

R=rsc
CC=golang-dev
https://golang.org/cl/207108

src/pkg/go/printer/nodes.go

@@ -671,6 +671,7 @@ func (p *printer) expr1(expr ast.Expr, prec1, depth int, ctxt exprContext, multi
 			// no parenthesis needed
 			p.print(x.Op)
 			if x.Op == token.RANGE {
+				// TODO(gri) Remove this code if it cannot be reached.
 				p.print(blank)
 			}
 			p.expr1(x.X, prec, depth, 0, multiLine)
@@ -1075,7 +1076,7 @@ func (p *printer) stmt(stmt ast.Stmt, multiLine *bool) {
 			p.expr(s.Value, multiLine)
 		}
 		p.print(blank, s.TokPos, s.Tok, blank, token.RANGE, blank)
-		p.expr(s.X, multiLine)
+		p.expr(stripParens(s.X), multiLine)
 		p.print(blank)
 		p.block(s.Body, 1, true)
 		*multiLine = true

src/pkg/go/printer/testdata/statements.golden

@@ -144,6 +144,9 @@ func _() {
 	for x := range []int{} {
 		use(x)
 	}
+	for x := range []int{} {
+		use(x)
+	}	// no parens printed
 }
 
 

src/pkg/go/printer/testdata/statements.input

@@ -107,6 +107,7 @@ func _() {
 	}
 	for x := expr;expr;expr = false { use(x) }
 	for x := range []int{} { use(x) }
+	for x := range (([]int{})) { use(x) }  // no parens printed
 }