cmd/compile: verify that rangefunc assigning to no vars works

cmd/compile: verify that rangefunc assigning to no vars works

This adds a test for
   for range seq2rangefunc { ... }
and
   for onevar := range seq2rangefunc { ... }

For #65236.

Change-Id: I083f8e4c19eb4ba0d6024d5314ac29d941141778
Reviewed-on: https://go-review.googlesource.com/c/go/+/596135
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>