cmd/compile: don't run ComputeAddrTaken on imported generic functions

It causes a crash because of the unexpected XDOT operation. It's not
needed, since we will run ComputeAddrTaken() on function instantiations
after stenciling. And it's not always correct, since we may not be able
to distinguish between a array and a slice, if a type is dependent on a
type param.

However, we do need to call ComputeAddrTaken on instantiations created
during inlining, since that is after the main ComputeAddrTaken pass.

Fixes #49659

Change-Id: I0bb610cf11f14e4aa9068f6ca2a012337b069c79
Reviewed-on: https://go-review.googlesource.com/c/go/+/365214


Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>