analyzer: add region_model_manager::get_or_create_int_cst

analyzer: add region_model_manager::get_or_create_int_cst

gcc/analyzer/ChangeLog:
	* region-model-manager.cc
	(region_model_manager::get_or_create_int_cst): New.
	(region_model_manager::maybe_undo_optimize_bit_field_compare): Use
	it to simplify away a local tree.
	* region-model.cc (region_model::on_setjmp): Likewise.
	(region_model::on_longjmp): Likewise.
	* region-model.h (region_model_manager::get_or_create_int_cst):
	New decl.
	* store.cc (binding_cluster::zero_fill_region): Use it to simplify
	away a local tree.

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