cmd/vet: simplify side effects func call logic
cmd/vet: simplify side effects func call logic

Instead of first looking for values of unnamed signature type, first
treat the types and builtins. All the remaining cases will be what we're
after.

Change-Id: I328e22ae0be1cccaeb45ed4ddaa360233d447e7e
Reviewed-on: https://go-review.googlesource.com/117835
Run-TryBot: Daniel Martí <29bf28b42a938ace12a5819da4ed4ba03d82f315@mvdan.cc>
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

josharian/go,josharian/go,cloudflare/go,josharian/go,cloudflare/go,nawawi/go,CAFxX/go,nawawi/go,akutz/go,CAFxX/go,golang/go,cloudflare/go,nawawi/go,akutz/go,golang/go,josharian/go,akutz/go,CAFxX/go,CAFxX/go,josharian/go,golang/go,cloudflare/go,akutz/go,cloudflare/go,josharian/go,josharian/go,akutz/go,nawawi/go,CAFxX/go,golang/go,golang/go,cloudflare/go,golang/go,akutz/go,akutz/go,CAFxX/go,CAFxX/go,CAFxX/go,cloudflare/go,golang/go,nawawi/go,golang/go,cloudflare/go,josharian/go,nawawi/go,akutz/go,nawawi/go,nawawi/go