reflect: a few microoptimizations

Replace i < 0 || i >= x with uint(i) >= uint(x).
Shorten a few other code sequences.
Move the kind bits to the bottom of the flag word, to avoid shifts.

LGTM=r
R=r, bradfitz
CC=golang-codereviews
https://golang.org/cl/159020043

src/reflect/type.go

@@ -490,7 +490,7 @@ func (t *uncommonType) Method(i int) (m Method) {
 	if p.name != nil {
 		m.Name = *p.name
 	}
-	fl := flag(Func) << flagKindShift
+	fl := flag(Func)
 	if p.pkgPath != nil {
 		m.PkgPath = *p.pkgPath
 		fl |= flagRO