[InstCombine] change 'not' match for bitwise select

The tests diffs are logically equivalent, and so this is
generally NFC, but this makes the code match the code
comment.

It should also be more efficient. If we choose the 'not'
operand (rather than the 'not' instruction) as the select
condition, then we don't have to invert the select
condition/operands as a subsequent transform.