[NFC] DI: Used already existing local variable.

Rather than repeating the same expression that already defines it.

Author: nate-chandler

lib/SILOptimizer/Mandatory/DefiniteInitialization.cpp

@@ -2607,7 +2607,7 @@ void LifetimeChecker::processUninitializedRelease(SILInstruction *Release,
   // If we see an alloc_box as the pointer, then we're deallocating a 'box' for
   // self. Make sure that the box gets deallocated (not released) since the
   // pointer it contains will be manually cleaned up.
-  auto *MUI = dyn_cast<MarkUninitializedInst>(Release->getOperand(0));
+  auto *MUI = dyn_cast<MarkUninitializedInst>(Pointer);
 
   if (MUI && isa<AllocBoxInst>(MUI->getOperand())) {
     Pointer = MUI->getSingleUserOfType<ProjectBoxInst>();