reflect: optimize (reflect.Type).Name

Improves JSON decoding on linux/amd64.

name                   old time/op    new time/op    delta
CodeUnmarshal-40         89.3ms ± 2%    86.3ms ± 2%  -3.31%  (p=0.000 n=22+22)

name                   old speed      new speed      delta
CodeUnmarshal-40       21.7MB/s ± 2%  22.5MB/s ± 2%  +3.44%  (p=0.000 n=22+22)

Updates #16117

Change-Id: I52acf31d7729400cfe6693e46292d41e1addba3d
Reviewed-on: https://go-review.googlesource.com/24410
Run-TryBot: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/reflect/type.go

@@ -891,25 +891,30 @@ func hasPrefix(s, prefix string) bool {
 
 func (t *rtype) Name() string {
 	s := t.String()
-	if hasPrefix(s, "map[") {
-		return ""
-	}
-	if hasPrefix(s, "struct {") {
-		return ""
-	}
-	if hasPrefix(s, "chan ") {
-		return ""
-	}
-	if hasPrefix(s, "chan<-") {
-		return ""
-	}
-	if hasPrefix(s, "func(") {
-		return ""
-	}
-	if hasPrefix(s, "interface {") {
-		return ""
-	}
 	switch s[0] {
+	case 'm':
+		if hasPrefix(s, "map[") {
+			return ""
+		}
+	case 's':
+		if hasPrefix(s, "struct {") {
+			return ""
+		}
+	case 'c':
+		if hasPrefix(s, "chan ") {
+			return ""
+		}
+		if hasPrefix(s, "chan<-") {
+			return ""
+		}
+	case 'f':
+		if hasPrefix(s, "func(") {
+			return ""
+		}
+	case 'i':
+		if hasPrefix(s, "interface {") {
+			return ""
+		}
 	case '[', '*', '<':
 		return ""
 	}