all: replace strings.Index with strings.Contains where possible

Change-Id: Ia613f1c37bfce800ece0533a5326fca91d99a66a
Reviewed-on: https://go-review.googlesource.com/18120
Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
diff --git a/src/cmd/compile/internal/gc/bexport.go b/src/cmd/compile/internal/gc/bexport.go
index b49f0fb..3edd048 100644
--- a/src/cmd/compile/internal/gc/bexport.go
+++ b/src/cmd/compile/internal/gc/bexport.go
@@ -877,7 +877,7 @@
 // tracef is like fmt.Printf but it rewrites the format string
 // to take care of indentation.
 func (p *exporter) tracef(format string, args ...interface{}) {
-	if strings.IndexAny(format, "<>\n") >= 0 {
+	if strings.ContainsAny(format, "<>\n") {
 		var buf bytes.Buffer
 		for i := 0; i < len(format); i++ {
 			// no need to deal with runes