IR: optimize IrType.isNullable, do not use DFS

DFS is not justified for type usages. In contrast to type hierarchies,
traversing the type usage naively with recursion has linear, not
exponential, complexity.

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/types/IrTypeUtils.kt

@@ -11,15 +11,11 @@ import org.jetbrains.kotlin.ir.symbols.FqNameEqualityChecker
 import org.jetbrains.kotlin.ir.symbols.IrClassSymbol
 import org.jetbrains.kotlin.ir.symbols.IrClassifierSymbol
 import org.jetbrains.kotlin.ir.symbols.IrTypeParameterSymbol
-import org.jetbrains.kotlin.ir.types.impl.IrSimpleTypeBuilder
-import org.jetbrains.kotlin.ir.types.impl.buildSimpleType
-import org.jetbrains.kotlin.ir.types.impl.buildTypeProjection
 import org.jetbrains.kotlin.ir.util.fqNameWhenAvailable
 import org.jetbrains.kotlin.ir.util.render
 import org.jetbrains.kotlin.resolve.calls.NewCommonSuperTypeCalculator
 import org.jetbrains.kotlin.types.AbstractTypeChecker
 import org.jetbrains.kotlin.types.AbstractTypeCheckerContext
-import org.jetbrains.kotlin.types.Variance
 import org.jetbrains.kotlin.utils.DFS
 
 fun IrClassifierSymbol.superTypes() = when (this) {
@@ -80,22 +76,16 @@ fun Collection<IrType>.commonSupertype(irBuiltIns: IrBuiltIns): IrType {
     }
 }
 
-fun IrType.isNullable(): Boolean = DFS.ifAny(
-    listOf(this),
-    {
-        when (val classifier = it.classifierOrNull) {
-            is IrTypeParameterSymbol -> classifier.owner.superTypes
-            is IrClassSymbol -> emptyList()
-            null -> emptyList()
+fun IrType.isNullable(): Boolean =
+    when (this) {
+        is IrSimpleType -> when (val classifier = classifier) {
+            is IrClassSymbol -> hasQuestionMark
+            is IrTypeParameterSymbol -> hasQuestionMark || classifier.owner.superTypes.any(IrType::isNullable)
             else -> error("Unsupported classifier: $classifier")
         }
-    }, {
-        when (it) {
-            is IrSimpleType -> it.hasQuestionMark
-            else -> it is IrDynamicType
-        }
+        is IrDynamicType -> true
+        else -> false
     }
-)
 
 val IrType.isBoxedArray: Boolean
     get() = classOrNull?.owner?.fqNameWhenAvailable == KotlinBuiltIns.FQ_NAMES.array.toSafe()