cmd/compile: better error message for invalid untyped nil conversion

In case of an invalid untyped nil conversion, the compiler's original
type checker leaves it to the caller to report a suitable error message.
But types2 does not, it always reports the invalid conversion.

CL 328053 made types2 report a better error message, and match the
original compiler behavior. But it ignored the case of untyped nil.

This CL adds that missing case, by checking whether the two operands can
be mixed when untyped nil is present.

Fixes #48784

Change-Id: Idc7d86eb0245aa18ca428e278f4416d6b3679058
Reviewed-on: https://go-review.googlesource.com/c/go/+/354049


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>