re PR c/28706 (Compile failure with --combine and explicitly aligned structures)
PR c/28706
PR c/28712
* tree.c (merge_attributes, attribute_list_contained): If both
TREE_VALUEs are TREE_LISTs, use simple_cst_list_equal instead of
simple_cst_equal.
* c-typeck.c (comptypes_internal): Don't consider aggregates
in different TUs as compatible if there one set of attributes is
not a subset of the other type's attributes.
(composite_type): Try harder not to create a new aggregate type.
* gcc.dg/pr28706.c: New test.
* gcc.dg/pr28712.c: New test.