unicode/utf8: minor code simplification

It's a little bit waste to check if r is not a surrogate
code point because RuneError is not a surrogate code point.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/79230043

src/pkg/unicode/utf8/utf8.go

@@ -329,37 +329,29 @@ func RuneLen(r rune) int {
 // It returns the number of bytes written.
 func EncodeRune(p []byte, r rune) int {
 	// Negative values are erroneous.  Making it unsigned addresses the problem.
-	if uint32(r) <= rune1Max {
+	switch i := uint32(r); {
+	case i <= rune1Max:
 		p[0] = byte(r)
 		return 1
-	}
-
-	if uint32(r) <= rune2Max {
+	case i <= rune2Max:
 		p[0] = t2 | byte(r>>6)
 		p[1] = tx | byte(r)&maskx
 		return 2
-	}
-
-	if uint32(r) > MaxRune {
+	case i > MaxRune, surrogateMin <= i && i <= surrogateMax:
 		r = RuneError
-	}
-
-	if surrogateMin <= r && r <= surrogateMax {
-		r = RuneError
-	}
-
-	if uint32(r) <= rune3Max {
+		fallthrough
+	case i <= rune3Max:
 		p[0] = t3 | byte(r>>12)
 		p[1] = tx | byte(r>>6)&maskx
 		p[2] = tx | byte(r)&maskx
 		return 3
+	default:
+		p[0] = t4 | byte(r>>18)
+		p[1] = tx | byte(r>>12)&maskx
+		p[2] = tx | byte(r>>6)&maskx
+		p[3] = tx | byte(r)&maskx
+		return 4
 	}
-
-	p[0] = t4 | byte(r>>18)
-	p[1] = tx | byte(r>>12)&maskx
-	p[2] = tx | byte(r>>6)&maskx
-	p[3] = tx | byte(r)&maskx
-	return 4
 }
 
 // RuneCount returns the number of runes in p.  Erroneous and short