Use a type switch instead of calling Val.Ctype (which in turn just
uses a type switch anyway).

Use continue statements to simplify the control flow.

Change-Id: I65c139d706d4d78e5b4ce09d1b1505a3e424496b
Reviewed-on: https://go-review.googlesource.com/21173Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>