Callers of irange_bitmask must normalize value/mask pairs.

As per the documentation, irange_bitmask must have the unknown bits in
the mask set to 0 in the value field.  Even though we say we must have
normalized value/mask bits, we don't enforce it, opting to normalize
on the fly in union and intersect.  Avoiding this lazy enforcing as
well as the extra saving/restoring involved in returning the changed
status, gives us a performance increase of 1.25% for VRP and 1.51% for
ipa-CP.

gcc/ChangeLog:

	* tree-ssa-ccp.cc (ccp_finalize): Normalize before calling
	set_bitmask.
	* value-range.cc (irange::intersect_bitmask): Calculate changed
	irange_bitmask bits on our own.
	(irange::union_bitmask): Same.
	(irange_bitmask::verify_mask): Verify that bits are normalized.
	* value-range.h (irange_bitmask::union_): Do not normalize.
	Remove return value.
	(irange_bitmask::intersect): Same.