Iterate over bits from most significant to least significant.

For the <code>i^th</code> bit, if both <code>a</code> and <code>b</code> have the same value, we can always make <code>x</code>’s <code>i^th</code> bit different from <code>a</code> and <code>b</code>, so <code>a ^ x</code> and <code>b ^ x</code> both have the <code>i^th</cod> bit set.

Otherwise, we can only set the <code>i^th</code> bit of one of <code>a ^ x</code> or <code>b ^ x</code>. Depending on the previous bits of <code>a ^ x</code> or <code>b ^ x</code>, we should set the smaller value’s <code>i^th</code> bit.