[range-ops] Avoid unnecessary intersection in update_known_bitmask.

[range-ops] Avoid unnecessary intersection in update_known_bitmask.

All the work for keeping the maybe nonzero masks up to date is being
done by the bit-CCP code now.  Any bitmask inherent in the range that
range-ops may have calculated has no extra information, so the
intersection is unnecessary.

gcc/ChangeLog:

	* range-op.cc (update_known_bitmask): Avoid unnecessary intersection.