bytes, strings: micro-optimize EqualFold

The first loop leaves the lengths of the two arguments unchanged.

Take advantage of this invariant in the loop's condition. Here are some
benchmark results (no change to allocations):

goos: darwin
goarch: amd64
pkg: strings
cpu: Intel(R) Core(TM) i7-6700HQ CPU @ 2.60GHz
                          │     old     │                new                 │
                          │   sec/op    │   sec/op     vs base               │
EqualFold/Tests-8           240.0n ± 4%   245.1n ± 5%       ~ (p=0.516 n=20)
EqualFold/ASCII-8           11.50n ± 1%   11.04n ± 0%  -3.96% (p=0.000 n=20)
EqualFold/UnicodePrefix-8   102.1n ± 0%   102.2n ± 0%       ~ (p=0.455 n=20)
EqualFold/UnicodeSuffix-8   90.14n ± 0%   89.80n ± 1%       ~ (p=0.113 n=20)
geomean                     71.00n        70.60n       -0.56%

src/bytes/bytes.go

@@ -1228,7 +1228,7 @@ func ReplaceAll(s, old, new []byte) []byte {
 func EqualFold(s, t []byte) bool {
 	// ASCII fast path
 	i := 0
-	for ; i < len(s) && i < len(t); i++ {
+	for n := min(len(s), len(t)); i < n; i++ {
 		sr := s[i]
 		tr := t[i]
 		if sr|tr >= utf8.RuneSelf {

src/strings/strings.go

@@ -1194,7 +1194,7 @@ func ReplaceAll(s, old, new string) string {
 func EqualFold(s, t string) bool {
 	// ASCII fast path
 	i := 0
-	for ; i < len(s) && i < len(t); i++ {
+	for n := min(len(s), len(t)); i < n; i++ {
 		sr := s[i]
 		tr := t[i]
 		if sr|tr >= utf8.RuneSelf {