[AsmPrinter] fix nullptr dereference for MBBs with hasAddressTaken property without BB

Basic block pointer is dereferenced unconditionally for MBBs with
hasAddressTaken property.

MBBs might have hasAddressTaken property without reference to BB.
Backend developers must assign fake BB to MBB to workaround this issue
and it should be fixed.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D108092

Author: ivafanas

llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

@@ -3270,21 +3270,21 @@ void AsmPrinter::emitBasicBlockStart(const MachineBasicBlock &MBB) {
   // reference the block.  It is possible that there is more than one label
   // here, because multiple LLVM BB's may have been RAUW'd to this block after
   // the references were generated.
+  const BasicBlock *BB = MBB.getBasicBlock();
   if (MBB.hasAddressTaken()) {
-    const BasicBlock *BB = MBB.getBasicBlock();
     if (isVerbose())
       OutStreamer->AddComment("Block address taken");
 
     // MBBs can have their address taken as part of CodeGen without having
     // their corresponding BB's address taken in IR
-    if (BB->hasAddressTaken())
+    if (BB && BB->hasAddressTaken())
       for (MCSymbol *Sym : MMI->getAddrLabelSymbolToEmit(BB))
         OutStreamer->emitLabel(Sym);
   }
 
   // Print some verbose block comments.
   if (isVerbose()) {
-    if (const BasicBlock *BB = MBB.getBasicBlock()) {
+    if (BB) {
       if (BB->hasName()) {
         BB->printAsOperand(OutStreamer->GetCommentOS(),
                            /*PrintType=*/false, BB->getModule());