Verify that reading back from vrange_storage doesn't drop bits.

Verify that reading back from vrange_storage doesn't drop bits.

We have a sanity check in the irange storage code to make sure that
reading back a cache entry we have just written to yields exactly the
same range.  There's no need to do this only for integers.  This patch
moves the code to a more generic place.

However, doing so tickles a latent bug in the frange code where a
range is being pessimized from [0.0, 1.0] to [-0.0, 1.0].  Exclude
checking frange's until this bug is fixed.

gcc/ChangeLog:

	* value-range-storage.cc (irange_storage::set_irange): Move
	verification code from here...
	(vrange_storage::set_vrange): ...to here.