Fix leak on foreach by ref assign to property

In this case we ended up creating an ASSIGN_OBJ_REF with VAR
result operand, which was not freed.

Fix this by implementing assign_r

Fix leak on foreach by ref assign to property

In this case we ended up creating an ASSIGN_OBJ_REF with VAR
result operand, which was not freed.

Fix this by implementing assign_ref_znode the same was as
assign_znode, i.e. performing an assignment with result and
then freeing the result, which will result mark the result as
UNUSED. This is more robust than the special handling for
result == NULL that was used before.

This fixes one of the issues reported in bug #81190.

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