IR: Simplify two loops walking ConstantDataSequential, NFC

[dexonsmith]

Summary

IR: Simplify two loops walking ConstantDataSequential, NFC

Follow-up to b2b7cf39d596b1528cd64015575b3f5d1461c011.

[dblaikie]

Looks good!

llvm/lib/IR/Constants.cpp

auto &Slot =
            .pImpl->CDSConstants.insert(std::make_pair(Elements, nullptr))
            .first;
 
   // The bucket can point to a linked list of different CDS's that have the same
   // body but different types.  For example, 0,0,0,1 could be a 4 element array
   // of i8, or a 1-element array of i32.  They'll both end up in the same
   /// StringMap bucket, linked up by their Next pointers.  Walk the list.
   std::unique_ptr<ConstantDataSequential> *Entry = &Slot.second;
-  for (ConstantDataSequential *Node = Entry->get(); Node;
-       Entry = &Node->Next, Node = Entry->get())
-    if (Node->getType() == Ty)
-      return Node;
+  for (; *Entry; Entry = &(*Entry)->Next)
+    if ((*Entry)->getType() == Ty)
+      return Entry->get();
 
   // Okay, we didn't get a hit.  Create a node of the right class, link it in,
   // and return it.
   if (isa<ArrayType>(Ty)) {
     // Use reset because std::make_unique can't access the constructor.
     Entry->reset(new ConstantDataArray(Ty, Slot.first().data()));
     return Entry->get();
   }
if (!(*Entry)->Next) {
     // entry, and removing the entry should remove the bucket completely.
     assert(Entry->get() == this && "Hash mismatch in ConstantDataSequential");
     getContext().pImpl->CDSConstants.erase(Slot);
     return;
   }
 
   // Otherwise, there are multiple entries linked off the bucket, unlink the
   // node we care about but keep the bucket around.
-  for (ConstantDataSequential *Node = Entry->get();;
-       Entry = &Node->Next, Node = Entry->get()) {
+  while (true) {
+    std::unique_ptr<ConstantDataSequential> &Node = *Entry;
     assert(Node && "Didn't find entry in its uniquing hash table!");
     // If we found our entry, unlink it from the list and we're done.
-    if (Node == this) {
-      *Entry = std::move(Node->Next);
+    if (Node.get() == this) {
+      Node = std::move(Node->Next);
       return;
     }
+
+    Entry = &Node->Next;
   }
 }
 
 /// getFP() constructors - Return a constant of array type with a float
 /// element type taken from argument `ElementType', and count taken from
 /// argument `Elts'.  The amount of bits of the contained type must match the
 /// number of bits of the type contained in the passed in ArrayRef.
 /// (i.e. half or bfloat for 16bits, float for 32bits, double for 64bits) Note