unicode/utf8: make FullRune inlinable

This has same readability and allows to inline FullRune for massive
performance gain:

FullASCIIRune-6                      4.36ns ± 0%  1.25ns ± 0%  -71.33%  (p=0.000 n=8+10)
FullJapaneseRune-6                   4.70ns ± 0%  1.42ns ± 1%  -69.68%  (p=0.000 n=9+10)

Change-Id: I95edd6292417a28aac244e40afb713596a087d93
Reviewed-on: https://go-review.googlesource.com/63332
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>

src/unicode/utf8/utf8.go

@@ -110,12 +110,10 @@ func FullRune(p []byte) bool {
 	}
 	// Must be short or invalid.
 	accept := acceptRanges[x>>4]
-	if n > 1 {
-		if c := p[1]; c < accept.lo || accept.hi < c {
-			return true
-		} else if n > 2 && (p[2] < locb || hicb < p[2]) {
-			return true
-		}
+	if n > 1 && (p[1] < accept.lo || accept.hi < p[1]) {
+		return true
+	} else if n > 2 && (p[2] < locb || hicb < p[2]) {
+		return true
 	}
 	return false
 }
@@ -132,12 +130,10 @@ func FullRuneInString(s string) bool {
 	}
 	// Must be short or invalid.
 	accept := acceptRanges[x>>4]
-	if n > 1 {
-		if c := s[1]; c < accept.lo || accept.hi < c {
-			return true
-		} else if n > 2 && (s[2] < locb || hicb < s[2]) {
-			return true
-		}
+	if n > 1 && (s[1] < accept.lo || accept.hi < s[1]) {
+		return true
+	} else if n > 2 && (s[2] < locb || hicb < s[2]) {
+		return true
 	}
 	return false
 }