unicode/utf8: reduce bounds checks in EncodeRune

Provide bounds elim hints in EncodeRune.

name                  old time/op  new time/op  delta
EncodeASCIIRune-4     2.69ns ± 2%  2.69ns ± 2%    ~     (p=0.193 n=47+46)
EncodeJapaneseRune-4  5.97ns ± 2%  5.38ns ± 2%  -9.93%  (p=0.000 n=49+50)

Change-Id: I1a6dcffff3bdd64ab93c2130021e3b00981de4c8
Reviewed-on: https://go-review.googlesource.com/28492Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/unicode/utf8/utf8.go

@@ -347,6 +347,7 @@ func EncodeRune(p []byte, r rune) int {
 		p[0] = byte(r)
 		return 1
 	case i <= rune2Max:
+		_ = p[1] // eliminate bounds checks
 		p[0] = t2 | byte(r>>6)
 		p[1] = tx | byte(r)&maskx
 		return 2
@@ -354,11 +355,13 @@ func EncodeRune(p []byte, r rune) int {
 		r = RuneError
 		fallthrough
 	case i <= rune3Max:
+		_ = p[2] // eliminate bounds checks
 		p[0] = t3 | byte(r>>12)
 		p[1] = tx | byte(r>>6)&maskx
 		p[2] = tx | byte(r)&maskx
 		return 3
 	default:
+		_ = p[3] // eliminate bounds checks
 		p[0] = t4 | byte(r>>18)
 		p[1] = tx | byte(r>>12)&maskx
 		p[2] = tx | byte(r>>6)&maskx