[X86] detectAVGPattern - use matchUnaryPredicate helper. NFC.

Use the ISD::matchUnaryPredicate helper to check for inrange constants.

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -42492,18 +42492,9 @@ static SDValue detectAVGPattern(SDValue In, EVT VT, SelectionDAG &DAG,
   // A lambda checking the given SDValue is a constant vector and each element
   // is in the range [Min, Max].
   auto IsConstVectorInRange = [](SDValue V, unsigned Min, unsigned Max) {
-    BuildVectorSDNode *BV = dyn_cast<BuildVectorSDNode>(V);
-    if (!BV || !BV->isConstant())
-      return false;
-    for (SDValue Op : V->ops()) {
-      ConstantSDNode *C = dyn_cast<ConstantSDNode>(Op);
-      if (!C)
-        return false;
-      const APInt &Val = C->getAPIntValue();
-      if (Val.ult(Min) || Val.ugt(Max))
-        return false;
-    }
-    return true;
+    return ISD::matchUnaryPredicate(V, [Min, Max](ConstantSDNode *C) {
+      return !(C->getAPIntValue().ult(Min) || C->getAPIntValue().ugt(Max));
+    });
   };
 
   // Check if each element of the vector is right-shifted by one.