Fix uninitialized variable warning. NFCI.

Author: RKSimon

llvm/lib/Target/SystemZ/SystemZLongBranch.cpp

@@ -162,7 +162,7 @@ class SystemZLongBranch : public MachineFunctionPass {
   void relaxBranches();
 
   const SystemZInstrInfo *TII = nullptr;
-  MachineFunction *MF;
+  MachineFunction *MF = nullptr;
   SmallVector<MBBInfo, 16> MBBs;
   SmallVector<TerminatorInfo, 16> Terminators;
 };