python: Fix issue when Self proxy resolves to a another proxy

The problem occurs when a proxy is being resolved to another proxy
that hasn't been resolved yet. The problematic case that was
triggering this issues in the VGIC. It was caused by parameters
looking a bit like this:

gic = Param.GicV2(Parent.any)
some_param = Param.Int(Self.gic.some_param)

When 'some_param' was resolved, it found the 'gic' parameter in
Self. However, that parameter hadn't been resolved yet, so the
existing code was setting the proxy evaluation context to the
unresolved Parent.any proxy without first unproxying it.

It seems like this bug depends on the graph traversal order and I have
so far only seen it when compiling gem5 with Python 3.

Change-Id: Iea12cc138765e70bfd6bb776b1efa012364db066
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/16004
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>

src/python/m5/proxy.py

@@ -187,13 +187,15 @@ class AttrProxy(BaseProxy):
             if hasattr(val, '_visited'):
                 visited = getattr(val, '_visited')
 
-            if not visited:
+            if visited:
+                return None, False
+
+            if not isproxy(val):
                 # for any additional unproxying to be done, pass the
                 # current, rather than the original object so that proxy
                 # has the right context
                 obj = val
-            else:
-                return None, False
+
         except:
             return None, False
         while isproxy(val):