[llvm] Add NCD search on Array of basic blocks (NFC)

llvm/include/llvm/Support/GenericDomTree.h

@@ -558,6 +558,22 @@ class DominatorTreeBase {
     return isPostDominator() && !A->getBlock();
   }
 
+  template <typename IteratorTy>
+  NodeT *findNearestCommonDominator(iterator_range<IteratorTy> Nodes) const {
+    assert(!Nodes.empty() && "Nodes list is empty!");
+
+    NodeT *NCD = *Nodes.begin();
+    for (NodeT *Node : llvm::drop_begin(Nodes)) {
+      NCD = findNearestCommonDominator(NCD, Node);
+
+      // Stop when the root is reached.
+      if (isVirtualRoot(getNode(NCD)))
+        return nullptr;
+    }
+
+    return NCD;
+  }
+
   //===--------------------------------------------------------------------===//
   // API to update (Post)DominatorTree information based on modifications to
   // the CFG...

llvm/lib/CodeGen/ShrinkWrap.cpp

@@ -375,12 +375,7 @@ bool ShrinkWrap::useOrDefCSROrFI(const MachineInstr &MI, RegScavenger *RS,
 template <typename ListOfBBs, typename DominanceAnalysis>
 static MachineBasicBlock *FindIDom(MachineBasicBlock &Block, ListOfBBs BBs,
                                    DominanceAnalysis &Dom, bool Strict = true) {
-  MachineBasicBlock *IDom = &Block;
-  for (MachineBasicBlock *BB : BBs) {
-    IDom = Dom.findNearestCommonDominator(IDom, BB);
-    if (!IDom)
-      break;
-  }
+  MachineBasicBlock *IDom = Dom.findNearestCommonDominator(iterator_range(BBs));
   if (Strict && IDom == &Block)
     return nullptr;
   return IDom;