FIR: Fix NI handling flexible type bounds

compiler/resolution/src/org/jetbrains/kotlin/resolve/calls/inference/components/ConstraintIncorporator.kt

@@ -231,9 +231,9 @@ private fun TypeSystemInferenceExtensionContext.getNestedArguments(type: KotlinT
     val stack = ArrayDeque<TypeArgumentMarker>()
 
     when (type) {
-        is FlexibleType -> {
-            stack.push(createTypeArgument(type.lowerBound, TypeVariance.INV))
-            stack.push(createTypeArgument(type.upperBound, TypeVariance.INV))
+        is FlexibleTypeMarker -> {
+            stack.push(createTypeArgument(type.lowerBound(), TypeVariance.INV))
+            stack.push(createTypeArgument(type.upperBound(), TypeVariance.INV))
         }
         else -> stack.push(createTypeArgument(type, TypeVariance.INV))
     }
@@ -253,9 +253,9 @@ private fun TypeSystemInferenceExtensionContext.getNestedArguments(type: KotlinT
         result.add(typeProjection)
 
         when (val projectedType = typeProjection.getType()) {
-            is FlexibleType -> {
-                addArgumentsToStack(projectedType.lowerBound)
-                addArgumentsToStack(projectedType.upperBound)
+            is FlexibleTypeMarker -> {
+                addArgumentsToStack(projectedType.lowerBound())
+                addArgumentsToStack(projectedType.upperBound())
             }
             else -> addArgumentsToStack(projectedType)
         }

compiler/testData/diagnostics/tests/generics/kt9203.fir.kt

@@ -7,5 +7,5 @@ import java.util.stream.IntStream
 fun main() {
     val xs = IntStream.range(0, 10).mapToObj { it.toString() }
             .collect(Collectors.toList())
-    <!UNRESOLVED_REFERENCE!>xs[0]<!>
+    xs[0]
 }