[SystemZ] lowerDYNAMIC_STACKALLOC_XPLINK - use cast<> instead of dyn_cast<> to avoid dereference of nullptr

The pointer is always dereferenced, so assert the cast is correct instead of returning nullptr

llvm/lib/Target/SystemZ/SystemZISelLowering.cpp

@@ -3577,7 +3577,7 @@ SystemZTargetLowering::lowerDYNAMIC_STACKALLOC_XPLINK(SDValue Op,
   // If user has set the no alignment function attribute, ignore
   // alloca alignments.
   uint64_t AlignVal =
-      (RealignOpt ? dyn_cast<ConstantSDNode>(Align)->getZExtValue() : 0);
+      (RealignOpt ? cast<ConstantSDNode>(Align)->getZExtValue() : 0);
 
   uint64_t StackAlign = TFI->getStackAlignment();
   uint64_t RequiredAlign = std::max(AlignVal, StackAlign);