Remove assert from set_nonzero_bits.

Remove assert from set_nonzero_bits.

The assert removed by this patch was there to keep users from passing
masks of incompatible types.  The self tests are passing host wide
ints down (set_nonzero_bits (-1)), which seem to be 32 bits, whereas
some embedded targets have integer_type_node's of 16-bits.  This is
causing problems in m32c-elf, among others.

I suppose there's no harm in passing a 32-bit mask, because
set_nonzero_bits calls wide_int::from() to convert the mask to the
appropriate type.  So we can remove the assert.

gcc/ChangeLog:

	* value-range.cc (irange::set_nonzero_bits): Remove assert.