analyzer: fix ICE on unreachable calls [PR 93947]

analyzer: fix ICE on unreachable calls [PR 93947]

PR analyzer/93947 reports an ICE at -O1 when attempting to analyze a
call that has been optimized away as unreachable.

The root cause is a NULL dereference due to the fndecl having a NULL
cgraph_node: the cgraph_node was created by
pass_build_cgraph_edges::execute, but was later removed by
symbol_table::remove_unreachable_nodes before the analyzer pass.

This patch fixes it by checking for NULL before handling the
cgraph_node.

The reproducer demonstrates a weakness in the analyzer's constraint
handling, where region_model::apply_constraints_for_gswitch fails
to spot when the cases fully cover the data type, and thus make the
default impossible.  For now this is xfail-ed in the testcase.

gcc/analyzer/ChangeLog:
	PR analyzer/93947
	* region-model.cc (region_model::get_fndecl_for_call): Gracefully
	fail for fn_decls that don't have a cgraph_node.

gcc/testsuite/ChangeLog:
	PR analyzer/93947
	* gcc.dg/analyzer/torture/pr93947.c: New test.