get_bitmask is sometimes less refined.

get_bitmask is sometimes less refined.

get_bitmask intersects the current mask with a mask generated from the
range.  If the 2 masks are incompatible, it currently returns UNKNOWN.
Instead, ti should return the original mask or information is lost.

	* value-range.cc (irange::get_bitmask): Return original mask if
	result is unknown.
	(assert_snap_result): New.
	(test_irange_snap_bounds): New.
	(range_tests_misc): Call test_irange_snap_bounds.