[AVR] When lowering Select8/Select16, put newly generated MBBs in the same

spot as the original MBB
(avr-rust/rust-legacy-fork#49)

Author: gergoerdi

lib/Target/AVR/AVRISelLowering.cpp

@@ -1612,8 +1612,9 @@ AVRTargetLowering::EmitInstrWithCustomInserter(MachineInstr &MI,
   MachineBasicBlock *trueMBB = MF->CreateMachineBasicBlock(LLVM_BB);
   MachineBasicBlock *falseMBB = MF->CreateMachineBasicBlock(LLVM_BB);
 
-  MachineFunction::iterator I = MBB->getParent()->begin();
-  ++I;
+  MachineFunction::iterator I;
+  for (I = MF->begin(); I != MF->end() && &(*I) != MBB; ++I);
+  if (I != MF->end()) ++I;
   MF->insert(I, trueMBB);
   MF->insert(I, falseMBB);