analyzer: fix equivalence class state purging [PR103533]

Whilst debugging state explosions seen when enabling taint detection
with -fanalyzer (PR analyzer/103533), I noticed that constraint
manager instances could contain stray, redundant constants, such
as this instance:

constraint_manager:
  equiv classes:
    ec0: {(int)0 == [m_constant]‘0’}
    ec1: {(size_t)4 == [m_constant]‘4’}
  constraints:

where there are two equivalence classes, each just containing a
constant, with no constraints using them.

This patch makes constraint_manager::canonicalize more aggressive
about purging state, handling the case of purging a redundant
EC containing just a constant.

gcc/analyzer/ChangeLog:
	PR analyzer/103533
	* constraint-manager.cc (equiv_class::contains_non_constant_p):
	New.
	(constraint_manager::canonicalize): Call it when determining
	redundant ECs.
	(selftest::test_purging): New selftest.
	(selftest::run_constraint_manager_tests): Likewise.
	* constraint-manager.h (equiv_class::contains_non_constant_p):
	New decl.

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