cmd/compile: remove confusing comment, fix comment for symExport

The symExport flag tells whether a symbol is in the export list
already or not (and it's also used to avoid being added to that
list). Exporting is based on that export list - no need to check
again.

Change-Id: I6056f97aa5c24a19376957da29199135c8da35f9
Reviewed-on: https://go-review.googlesource.com/39033Reviewed-by: Dave Cheney <dave@cheney.net>

src/cmd/compile/internal/gc/bexport.go

@@ -265,11 +265,6 @@ func export(out *bufio.Writer, trace bool) int {
 			Fatalf("exporter: unexpected symbol: %v", sym)
 		}
 
-		// TODO(gri) Should we do this check?
-		// if !sym.Export() {
-		// 	continue
-		// }
-
 		if sym.Def == nil {
 			Fatalf("exporter: unknown export symbol: %v", sym)
 		}
@@ -332,11 +327,6 @@ func export(out *bufio.Writer, trace bool) int {
 			Fatalf("exporter: unexpected symbol: %v", sym)
 		}
 
-		// TODO(gri) Should we do this check?
-		// if !sym.Export() {
-		// 	continue
-		// }
-
 		if sym.Def == nil {
 			Fatalf("exporter: unknown export symbol: %v", sym)
 		}

src/cmd/compile/internal/gc/go.go

@@ -61,7 +61,7 @@ type Sym struct {
 }
 
 const (
-	symExport = 1 << iota // to be exported
+	symExport = 1 << iota // added to exportlist (no need to add again)
 	symPackage
 	symExported // already written out by export
 	symUniq