strings: use Go style character range comparison in ToUpper/ToLower

As noted by Brad in CL 170954 for package bytes.

Change-Id: I2772a356299e54ba5b7884d537e6649039adb9be
Reviewed-on: https://go-review.googlesource.com/c/go/+/171198
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/strings/strings.go

@@ -559,7 +559,7 @@ func ToUpper(s string) string {
 			isASCII = false
 			break
 		}
-		hasLower = hasLower || (c >= 'a' && c <= 'z')
+		hasLower = hasLower || ('a' <= c && c <= 'z')
 	}
 
 	if isASCII { // optimize for ASCII-only strings.
@@ -570,7 +570,7 @@ func ToUpper(s string) string {
 		b.Grow(len(s))
 		for i := 0; i < len(s); i++ {
 			c := s[i]
-			if c >= 'a' && c <= 'z' {
+			if 'a' <= c && c <= 'z' {
 				c -= 'a' - 'A'
 			}
 			b.WriteByte(c)
@@ -589,7 +589,7 @@ func ToLower(s string) string {
 			isASCII = false
 			break
 		}
-		hasUpper = hasUpper || (c >= 'A' && c <= 'Z')
+		hasUpper = hasUpper || ('A' <= c && c <= 'Z')
 	}
 
 	if isASCII { // optimize for ASCII-only strings.
@@ -600,7 +600,7 @@ func ToLower(s string) string {
 		b.Grow(len(s))
 		for i := 0; i < len(s); i++ {
 			c := s[i]
-			if c >= 'A' && c <= 'Z' {
+			if 'A' <= c && c <= 'Z' {
 				c += 'a' - 'A'
 			}
 			b.WriteByte(c)