cmd/compile: in typ0(), load base type before checking s.Def

cmd/compile: in typ0(), load base type before checking s.Def

The loading of the base type in typ0() may cause s.Def to be defined for
the instantiated type, so load the base type before checking s.Def.

Fixes #50486

Change-Id: Ic039bc8f774dda534f4ccd1f920220b7a10dede6
Reviewed-on: https://go-review.googlesource.com/c/go/+/377094
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>