analyzer: fix false positives from -Wanalyzer-tainted-divisor [PR106225]

analyzer: fix false positives from -Wanalyzer-tainted-divisor [PR106225]

gcc/analyzer/ChangeLog:
	PR analyzer/106225
	* sm-taint.cc (taint_state_machine::on_stmt): Move handling of
	assignments from division to...
	(taint_state_machine::check_for_tainted_divisor): ...this new
	function.  Reject warning when the divisor is known to be non-zero.
	* sm.cc: Include "analyzer/program-state.h".
	(sm_context::get_old_region_model): New.
	* sm.h (sm_context::get_old_region_model): New decl.

gcc/testsuite/ChangeLog:
	PR analyzer/106225
	* gcc.dg/analyzer/taint-divisor-1.c: Add test coverage for various
	correct and incorrect checks against zero.

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