Start using KnownBits::computeForAddSub in

SelectionDAG::computeKnownBits when doing value

tracking for addition/subtraction.

This should improve the precision of the known bits,

as we only used to make a simple estimate of known

zeroes. The KnownBits::computeForAddSub function is

also able to deduce bits that are known to be one

in the result.