go/ast: minor cleanup

It's better to use IsValid() then checking a (possibly
partially set up) position against NoPos directly.

R=dsymonds
CC=golang-dev
https://golang.org/cl/6855099

src/pkg/go/ast/import.go

@@ -20,7 +20,7 @@ func SortImports(fset *token.FileSet, f *File) {
 			break
 		}
 
-		if d.Lparen == token.NoPos {
+		if !d.Lparen.IsValid() {
 			// Not a block: sorted by default.
 			continue
 		}