Optimize IF(a,IF(a,b,c),d) to IF(a,b,d)

Various guises this can occur in - often the result of nested attributes, each of which is checking if a condition is true.  Sometimes it may be possible to spot it in the optimizer.

Is it ever worse to make that transformation?  I don't think so because you are never creating an extra compare - even if the inner IF is still referenced.