[x86] use cast instead of dyn_cast for unchecked usage; NFC

This was noted as an independent clean-up in D112464.

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -5050,7 +5050,7 @@ static bool MayFoldLoadIntoBroadcastFromMem(SDValue Op, MVT EltVT,
 
   // We can not replace a wide volatile load with a broadcast-from-memory,
   // because that would narrow the load, which isn't legal for volatiles.
-  const LoadSDNode *Ld = dyn_cast<LoadSDNode>(Op.getNode());
+  auto *Ld = cast<LoadSDNode>(Op.getNode());
   return !Ld->isVolatile() ||
          Ld->getValueSizeInBits(0) == EltVT.getScalarSizeInBits();
 }