[AVR] When lowering shifts into loops, put newly generated MBBs in the same

spot as the original MBB (avr-rust/rust-legacy-fork#62)

Author: gergoerdi

lib/Target/AVR/AVRISelLowering.cpp

@@ -1471,7 +1471,10 @@ MachineBasicBlock *AVRTargetLowering::insertShift(MachineInstr &MI,
   }
 
   const BasicBlock *LLVM_BB = BB->getBasicBlock();
-  MachineFunction::iterator I = BB->getParent()->begin();
+
+  MachineFunction::iterator I;
+  for (I = F->begin(); I != F->end() && &(*I) != BB; ++I);
+  if (I != F->end()) ++I;
   ++I;
 
   // Create loop block.