analyzer: fix skipping of debug stmts [PR113253]

analyzer: fix skipping of debug stmts [PR113253]

PR analyzer/113253 reports a case where the analyzer output varied
with and without -g enabled.

The root cause was that debug stmts were in the
FOR_EACH_IMM_USE_FAST list for SSA names, leading to the analyzer's
state purging logic differing between the -g and non-debugging cases,
and thus leading to differences in the exploration of the user's code.

Fix by skipping such stmts in the state-purging logic, and removing
debug stmts when constructing the supergraph.

gcc/analyzer/ChangeLog:
	PR analyzer/113253
	* region-model.cc (region_model::on_stmt_pre): Add gcc_unreachable
	for debug statements.
	* state-purge.cc
	(state_purge_per_ssa_name::state_purge_per_ssa_name): Skip any
	debug stmts in the FOR_EACH_IMM_USE_FAST list.
	* supergraph.cc (supergraph::supergraph): Don't add debug stmts
	to the supernodes.

gcc/testsuite/ChangeLog:
	PR analyzer/113253
	* gcc.dg/analyzer/deref-before-check-pr113253.c: New test.

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