Record nonzero bits in the irange_bitmask of POLY_INT_CSTs

At the moment, ranger punts entirely on POLY_INT_CSTs.  Numerical
ranges are a bit difficult, unless we do start modelling bounds on
the indeterminates.  But we can at least track the nonzero bits.

gcc/
	* value-query.cc (range_query::get_tree_range): Use get_nonzero_bits
	to populate the irange_bitmask of a POLY_INT_CST.

gcc/testsuite/
	* gcc.target/aarch64/sve/cnt_fold_6.c: New test.