analyzer: fix uninitialized bitmap [PR112955]

In r14-5566-g841008d3966c0f I added a new ctor for
feasibility_state, but failed to call bitmap_clear
on m_snodes_visited.

Fixed thusly.

gcc/analyzer/ChangeLog:
	PR analyzer/112955
	* engine.cc (feasibility_state::feasibility_state): Initialize
	m_snodes_visited.

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