Add back functionality removed in r210497.

Instead of asserting, output a message stating that a null pointer was found.

llvm-svn: 211430
diff --git a/llvm/lib/Analysis/RegionPass.cpp b/llvm/lib/Analysis/RegionPass.cpp
index d11b332..71de144 100644
--- a/llvm/lib/Analysis/RegionPass.cpp
+++ b/llvm/lib/Analysis/RegionPass.cpp
@@ -196,8 +196,10 @@
   bool runOnRegion(Region *R, RGPassManager &RGM) override {
     Out << Banner;
     for (const auto &BB : R->blocks()) {
-      assert(BB != nullptr && "Expecting non-null Block");
-      BB->print(Out);
+      if (BB)
+        BB->print(Out);
+      else
+        Out << "Printing <null> Block";
     }
 
     return false;