analyzer: fixes to __atomic_{exchange,load,store} [PR114286]

In r14-1497-gef768035ae8090 I added some support to the analyzer for
__atomic_ builtins (enough to fix false positives I was seeing in
my integration tests).

Unfortunately I messed up the implementation of
__atomic_{exchange,load,store}, leading to ICEs seen in
PR analyzer/114286.

Fixed thusly, fixing the ICEs.  Given that we're in stage 4, the patch
doesn't add support for any of the various __atomic_compare_exchange
builtins, so that these continue to fall back to the analyzer's
"anything could happen" handling of unknown functions.

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

gcc/analyzer/ChangeLog:
	PR analyzer/114286
	* kf.cc (class kf_atomic_exchange): Reimplement based on signature
	seen in gimple, rather than user-facing signature.
	(class kf_atomic_load): Likewise.
	(class kf_atomic_store): New.
	(register_atomic_builtins): Register kf_atomic_store.

gcc/testsuite/ChangeLog:
	PR analyzer/114286
	* c-c++-common/analyzer/atomic-builtins-pr114286.c: New test.

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