improve IID reduction
signature = 8321484
Revert "improve IID reduction"

This reverts commit 2abf3f9cb50df66f8ded112266e41f30c179ef78.
Revert "IID at non PV nodes: depth-4 i/o depth/2"

More proper testing shows this was useless, and potentially regressive.

This reverts commit d420f3c994b9056d0a9a945064a96f71f09c3404.