unicode/utf8: optimize ValidRune

Re-writing the switch statement as a single boolean expression
reduces the number of branches that the compiler generates.
It is also arguably easier to read as a pair of numeric ranges
that valid runes can exist in.

No test changes since the existing test does a good job of
testing all of the boundaries.

This change was to gain back some performance after a correctness
fix done in http://golang.org/cl/32123.

The correctness fix (CL/32123) slowed down the benchmarks slightly:
	benchmark                   old ns/op     new ns/op     delta
	BenchmarkIndexRune/10-4     19.3          21.6          +11.92%
	BenchmarkIndexRune/32-4     33.6          35.2          +4.76%

Since the fix relies on utf8.ValidRune, this CL improves benchmarks:
	benchmark                   old ns/op     new ns/op     delta
	BenchmarkIndexRune/10-4     21.6          20.0          -7.41%
	BenchmarkIndexRune/32-4     35.2          33.5          -4.83%

Change-Id: Ib1ca10a2e29c90e879a8ef9b7221c33e85d015d8
Reviewed-on: https://go-review.googlesource.com/32122
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/unicode/utf8/utf8.go

@@ -516,12 +516,10 @@ func ValidString(s string) bool {
 // Code points that are out of range or a surrogate half are illegal.
 func ValidRune(r rune) bool {
 	switch {
-	case r < 0:
-		return false
-	case surrogateMin <= r && r <= surrogateMax:
-		return false
-	case r > MaxRune:
-		return false
+	case 0 <= r && r < surrogateMin:
+		return true
+	case surrogateMax < r && r <= MaxRune:
+		return true
 	}
-	return true
+	return false
 }