Revert "Recommit "[GlobalOpt] Pass DTU to removeUnreachableBlocks instead of recomputing.""

This reverts commit a03d7b0f.

As discussed in D68298, this causes a compile-time regression, in case
the DTs requested are not used elsewhere in GlobalOpt. We should only
get the DTs if they are available here, but this seems not possible with
the legacy pass manager from a module pass.