Clear nonzero mask when inverting ranges.

Clear nonzero mask when inverting ranges.

Every time we set a range we should take into account the nonzero
mask.  This happens automatically for the set() methods, plus all the
other assignment, intersect, and union methods.  Unfortunately I
forgot about the invert code.

Also, for good measure I audited the rest of the setters in
value_range.cc and plugged the legacy code to pessimize the masks to
-1 for union/intersect, since we don't support the masks on them (or
rather, we don't keep very good track of them).

Tested on x86-64 Linux.

gcc/ChangeLog:

	* value-range.cc (irange::copy_to_legacy): Set nonzero mask.
	(irange::legacy_intersect): Clear nonzero mask.
	(irange::legacy_union): Same.
	(irange::invert): Same.