This patch addresses 66270, another case where may_alias disrupted the canonical 
type system.  We ICE as TYPE_CANONICALs differ, but comptypes think they are the 
same.

There seems to be a bit of confusion as to whether pointers that differ only in 
TYPE_REF_CAN_ALIAS_ALL are the same canonical type or not.

Firstly, in tree.c build_pointer_type_for_mode, when the pointed-to type is not 
its own canonical type, that means the newly constructed pointer type is 
(possibly) not canonical either.  So we explicitly build a canonical type with:

   else if (TYPE_CANONICAL (to_type) != to_type)
     TYPE_CANONICAL (t)
       = build_reference_type_for_mode (TYPE_CANONICAL (to_type),
				       mode, false);

But we're passing 'false' in as 'can_alias_all', rather than pass the value 
passed into us.  That'll make a difference if the caller passed in true and 
to_type doesn't have may_alias set.  This is inconsistent at least, because we 
could sometimes end up with canonical types with T_R_C_A_A set (to-type is 
canonical) and sometimes with it not set.  It seems the right solution is to 
consider T_R_C_A_A as a distinguisher, thus we should pass can_alias_all to the 
canonical type builder.  Note that it is ok to pass the possibly modified 
can_alias_all in, and not the incoming value, because we only ever modify it to 
make it true -- and in that case the same behavior would happen in the canonical 
type builder because to_type and TYPE_CANONICAL (to_type) should have the same 
may_alias attribute.

Anyway, that's a bit of collateral confusion I fell over investigating.  With 
that out of the way, we have  to teach comptypes that T_R_C_A_A affects pointer 
type equality.  Hence add such a check to POINTER_TYPE case there.

bootstrapped on x86-linux & tested, ok?

nathan