[CodeGen] Fix a warning

This patch fixes:

  llvm/lib/CodeGen/MachineSink.cpp:1667:22: error: unused variable
  'Preheader' [-Werror,-Wunused-variable]

Author: kazutakahirata

llvm/lib/CodeGen/MachineSink.cpp

@@ -1664,8 +1664,7 @@ bool MachineSinking::aggressivelySinkIntoCycle(
     return false;
 
   LLVM_DEBUG(dbgs() << "AggressiveCycleSink: Finding sink block for: " << I);
-  MachineBasicBlock *Preheader = Cycle->getCyclePreheader();
-  assert(Preheader && "Cycle sink needs a preheader block");
+  assert(Cycle->getCyclePreheader() && "Cycle sink needs a preheader block");
   SmallVector<std::pair<RegSubRegPair, MachineInstr *>> Uses;
 
   MachineOperand &DefMO = I.getOperand(0);