Blocks with UnreachableInst terminators are considered as root nodes in
the PDT. This pessimize DSE, if there are no aliasing reads from the
potentially dead store and the block with the unreachable terminator.
If the PDT has a virtual root node and any of the root nodes has
UnreachableInst as terminator, fall back to the CFG scan, even the
common dominator of all killing blocks does not post-dominate the block
with potentially dead store.
It looks like the compile-time impact for the extra scans is negligible.