cmd/compile: look for function in instantiations in all global assignments

Add in some missing global assignment ops to the list of globals ops
that should be traversed to look for generic function instantiations.
The most common other one for global assigments (and the relevant one
for this bug) is OAS2FUNC, but also look at global assigments with
OAS2DOTTYPE, OAS2MAPR, OAS2RECV, and OASOP.

Bonus small fix: get rid of -G=3 case in ir.IsAddressable. Now that we
don't call the old typechecker from noder2, we don't need this -G-3
check anymore.

Fixes #45547.

Change-Id: I75fecec55ea0d6f62e1c2294d4d77447ed9be6ae
Reviewed-on: https://go-review.googlesource.com/c/go/+/310210


Trust: Dan Scales <danscales@google.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>