cmd/compile/internal/gc: simplify typechecking definitions

There are only a handful of nodes that we need to pass to
typecheckdef (OLITERAL, ONAME, OTYPE, and ONONAME), but typecheck1
takes the awkward approach of calling typecheckdef on every node with
Sym != nil, and then excluding a long list of uninteresting Ops that
have a non-nil Sym.

Passes toolstash-check.

Change-Id: I0271d2faff0208ad57ddc1f1a540a5fbed870234
Reviewed-on: https://go-review.googlesource.com/c/142657


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>