unicode/utf16: speed up and clean up Decode

name                        old time/op  new time/op  delta
DecodeValidASCII-4          94.7ns ± 1%  87.4ns ± 1%  -7.71%  (p=0.000 n=10+9)
DecodeValidJapaneseChars-4  91.0ns ± 2%  84.8ns ± 0%  -6.77%  (p=0.000 n=9+10)
DecodeRune-4                16.5ns ± 0%  16.6ns ± 2%    ~     (p=0.108 n=9+10)

For #6957

Change-Id: I618c15c2a42ef7ec6a5cd163b7c3f1a65ca4ad01
Reviewed-on: https://go-review.googlesource.com/19826Reviewed-by: Rob Pike <r@golang.org>

src/unicode/utf16/utf16.go

@@ -36,7 +36,7 @@ func IsSurrogate(r rune) bool {
 // the Unicode replacement code point U+FFFD.
 func DecodeRune(r1, r2 rune) rune {
 	if surr1 <= r1 && r1 < surr2 && surr2 <= r2 && r2 < surr3 {
-		return (r1-surr1)<<10 | (r2 - surr2) + 0x10000
+		return (r1-surr1)<<10 | (r2 - surr2) + surrSelf
 	}
 	return replacementChar
 }
@@ -88,21 +88,19 @@ func Decode(s []uint16) []rune {
 	n := 0
 	for i := 0; i < len(s); i++ {
 		switch r := s[i]; {
+		case r < surr1, surr3 <= r:
+			// normal rune
+			a[n] = rune(r)
 		case surr1 <= r && r < surr2 && i+1 < len(s) &&
 			surr2 <= s[i+1] && s[i+1] < surr3:
 			// valid surrogate sequence
 			a[n] = DecodeRune(rune(r), rune(s[i+1]))
 			i++
-			n++
-		case surr1 <= r && r < surr3:
+		default:
 			// invalid surrogate sequence
 			a[n] = replacementChar
-			n++
-		default:
-			// normal rune
-			a[n] = rune(r)
-			n++
 		}
+		n++
 	}
-	return a[0:n]
+	return a[:n]
 }