bytes, strings: update s390x code to match amd64 changes

Updates the s390x-specific files in these packages with the changes
to the amd64-specific files made during the review of CL 31690. I'd
like to keep these files in sync unless there is a reason to
diverge.

Change-Id: Id83e5ce11a45f877bdcc991d02b14416d1a2d8d2
Reviewed-on: https://go-review.googlesource.com/32574Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/bytes/bytes_s390x.go

@@ -32,6 +32,13 @@ func Index(s, sep []byte) int {
 		return 0
 	case n == 1:
 		return IndexByte(s, sep[0])
+	case n == len(s):
+		if Equal(sep, s) {
+			return 0
+		}
+		return -1
+	case n > len(s):
+		return -1
 	case n <= shortStringLen:
 		// Use brute force when s and sep both are small
 		if len(s) <= 64 {
@@ -68,13 +75,6 @@ func Index(s, sep []byte) int {
 			}
 		}
 		return -1
-	case n == len(s):
-		if Equal(sep, s) {
-			return 0
-		}
-		return -1
-	case n > len(s):
-		return -1
 	}
 	// Rabin-Karp search
 	hashsep, pow := hashStr(sep)

src/strings/strings_s390x.go

@@ -32,6 +32,13 @@ func Index(s, sep string) int {
 		return 0
 	case n == 1:
 		return IndexByte(s, sep[0])
+	case n == len(s):
+		if sep == s {
+			return 0
+		}
+		return -1
+	case n > len(s):
+		return -1
 	case n <= shortStringLen:
 		// Use brute force when s and sep both are small
 		if len(s) <= 64 {
@@ -68,13 +75,6 @@ func Index(s, sep string) int {
 			}
 		}
 		return -1
-	case n == len(s):
-		if sep == s {
-			return 0
-		}
-		return -1
-	case n > len(s):
-		return -1
 	}
 	// Rabin-Karp search
 	hashsep, pow := hashStr(sep)