Cache irange::num_pairs() for non-legacy code.

Cache irange::num_pairs() for non-legacy code.

This does for num_pairs() what my previous patch did for VR_UNDEFINED
and VR_VARYING.

Note that VR_ANTI_RANGE for legacy is always set to 2 ranges.  There
is only one way of representing a range, so a range that can be
represented as a VR_RANGE will never have a kind of VR_ANTI_RANGE.
Also legacy symbolics can also use VR_ANTI_RANGE, but no one will ever
ask for the bounds of such range, so m_num_ranges is irrelevant.

gcc/ChangeLog:

	* value-range.cc (irange::legacy_num_pairs): Remove.
	(irange::invert): Change gcc_assert to gcc_checking_assert.
	* value-range.h (irange::num_pairs): Adjust for a cached
	num_pairs().  Also, rename all gcc_assert's to
	gcc_checking_assert's.