reflect: optimize directlyAssignable to avoid rtype.Name call
directlyAssignable invoked rtype.Name() just to compare its result
to empty string. We really only need to check whether rtype has
name. It can be done much cheaper, by checking tflagNamed.
Benchmark: https://play.golang.org/p/V2BzESPuf2w
name old time/op new time/op delta
DirectlyAssignable-12 32.7ns ± 6% 6.6ns ± 6% -79.80% (p=0.008 n=5+5)