analyzer: don't create bindings or binding keys for empty regions [PR107882]

analyzer: don't create bindings or binding keys for empty regions [PR107882]

PR analyzer/107882 reports an ICE, due to trying to get a compound svalue
for this binding:

  cluster for: a:
    key:   {bytes 0-3}
    value:  {UNKNOWN()}
    key:   {empty}
    value:  {UNKNOWN()}
    key:   {bytes 4-7}
    value:  {UNKNOWN()}

where there's an binding to the unknown value of zero bits in size
"somewhere" within "a" (perhaps between bits 3 and 4?)

This makes no sense, so this patch adds an assertion that we never
attempt to create a binding key for an empty region, and adds early
rejection of attempts to get or set the values of such regions, fixing
the ICE.

gcc/analyzer/ChangeLog:
	PR analyzer/107882
	* region-model.cc (region_model::get_store_value): Return an
	unknown value for empty regions.
	(region_model::set_value): Bail on empty regions.
	* region.cc (region::empty_p): New.
	* region.h (region::empty_p): New decl.
	* state-purge.cc (same_binding_p): Bail if either region is empty.
	* store.cc (binding_key::make): Assert that a concrete binding's
	bit_size must be > 0.
	(binding_cluster::mark_region_as_unknown): Bail on empty regions.
	(binding_cluster::get_binding): Likewise.
	(binding_cluster::remove_overlapping_bindings): Likewise.
	(binding_cluster::on_unknown_fncall): Don't conjure values for
	empty regions.
	(store::fill_region): Bail on empty regions.
	* store.h (class concrete_binding): Update comment to reflect that
	the range of bits must be non-empty.
	(concrete_binding::concrete_binding): Assert that bit range is
	non-empty.

gcc/testsuite/ChangeLog:
	PR analyzer/107882
	* gcc.dg/analyzer/memcpy-pr107882.c: New test.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>