Revert "Recommit "[GlobalOpt] Pass DTU to removeUnreachableBlocks ins…

…tead of recomputing.""

This reverts commit a03d7b0.

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.

llvm/lib/Transforms/IPO/GlobalOpt.cpp

@@ -2298,10 +2298,14 @@ OptimizeFunctions(Module &M,
     // So, remove unreachable blocks from the function, because a) there's
     // no point in analyzing them and b) GlobalOpt should otherwise grow
     // some more complicated logic to break these cycles.
+    // Removing unreachable blocks might invalidate the dominator so we
+    // recalculate it.
     if (!F->isDeclaration()) {
-      auto &DT = LookupDomTree(*F);
-      DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy);
-      Changed |= removeUnreachableBlocks(*F, &DTU);
+      if (removeUnreachableBlocks(*F)) {
+        auto &DT = LookupDomTree(*F);
+        DT.recalculate(*F);
+        Changed = true;
+      }
     }
 
     Changed |= processGlobal(*F, GetTLI, LookupDomTree);