Apply suggestions from code review

Co-authored-by: Zhongpeng Lin <zplin@uber.com>

go/tools/builders/compilepkg.go

@@ -179,7 +179,7 @@ func compileArchive(
 
 	// This filter defines how generated source code should be treated by nogo.
 	//
-	// Use this to generated source file could be mapped back to original source.
+	// Use this to map generated source file back to original source.
 	// Assign an empty string value to exclude generated source from nogo run.
 	nogoSrcsTranslate := make(map[string]string)
 
@@ -369,10 +369,8 @@ func compileArchive(
 	outFactsPath := filepath.Join(workDir, nogoFact)
 	nogoSrcs := make([]string, 0, len(goSrcs))
 	for _, goSrc := range goSrcs {
-		if v, ok := nogoSrcsTranslate[goSrc]; ok {
-			if v != "" {
-				nogoSrcs = append(nogoSrcs, v)
-			}
+		if originSrc, ok := nogoSrcsTranslate[goSrc]; ok && originSrc != ""  {
+			nogoSrcs = append(nogoSrcs, originSrc)
 			continue
 		}