pass: "instruction should follow a label" info (#115)

Add the label in question to the error message.

pass/cfg.go

@@ -17,7 +17,7 @@ func LabelTarget(fn *ir.Function) error {
 		switch n := node.(type) {
 		case ir.Label:
 			if pending != empty {
-				return errors.New("instruction should follow a label")
+				return fmt.Errorf("expected instruction following label %q", pending)
 			}
 			pending = n
 			if _, found := target[pending]; found {

pass/cfg_test.go

@@ -66,7 +66,7 @@ func TestLabelTargetInstructionFollowLabel(t *testing.T) {
 
 	err := LabelTarget(f)
 
-	if err == nil || err.Error() != "instruction should follow a label" {
+	if err == nil || err.Error() != "expected instruction following label \"lblA\"" {
 		t.Fatalf("expected error when label is not followed by instruction; got %v", err)
 	}
 }