[MSSA] Print memory phis when inspecting walker.

This makes the MemorySSA and MemorySSA Walker printers consistent.
Invokation `-print<memoryssa-walker>` should also have the MemoryPhis.

llvm/lib/Analysis/MemorySSA.cpp

@@ -130,6 +130,12 @@ class MemorySSAWalkerAnnotatedWriter : public AssemblyAnnotationWriter {
   MemorySSAWalkerAnnotatedWriter(MemorySSA *M)
       : MSSA(M), Walker(M->getWalker()) {}
 
+  void emitBasicBlockStartAnnot(const BasicBlock *BB,
+                                formatted_raw_ostream &OS) override {
+    if (MemoryAccess *MA = MSSA->getMemoryAccess(BB))
+      OS << "; " << *MA << "\n";
+  }
+
   void emitInstructionAnnot(const Instruction *I,
                             formatted_raw_ostream &OS) override {
     if (MemoryAccess *MA = MSSA->getMemoryAccess(I)) {