[ADCE] Don't mark blocks with UnreachableInst as live

[kuhar]

Summary

Both ReturnInst and UnreachableInst exit functions, so we can treat them the same in ADCE and not mark them as live.
The only the CFG nodes corresponding to non-trivial PostDominatorTree roots (i.e. infinite loop blocks) need to be marked as live here for ADCE to work correctly.

lib/Transforms/Scalar/ADCE.cpp

void AggressiveDeadCodeElimination::initialize() {
   // Mark blocks live if there is no path from the block to a
   // return of the function.
   // We do this by seeing which of the postdomtree root children exit the
   // program, and for all others, mark the subtree live.
   for (auto &PDTChild : children<DomTreeNode *>(PDT.getRootNode())) {
     auto *BB = PDTChild->getBlock();
     auto &Info = BlockInfo[BB];
     // Real function return
-    if (isa<ReturnInst>(Info.Terminator)) {
-      DEBUG(dbgs() << "post-dom root child is a return: " << BB->getName()
-                   << '\n';);
+    if (isa<ReturnInst>(Info.Terminator) ||
+        isa<UnreachableInst>(Info.Terminator)) {
+      DEBUG(dbgs() << "post-dom root child is a return or unreachable: "
+                   << BB->getName() << '\n';);
       continue;
     }
 
     // This child is something else, like an infinite loop.
     for (auto DFNode : depth_first(PDTChild))
       markLive(BlockInfo[DFNode->getBlock()].Terminator);
   }