FIR2IR: add implicit NOT_NULL cast for @FlexibleNullability type

compiler/fir/tree/src/org/jetbrains/kotlin/fir/types/FirTypeUtils.kt

@@ -138,3 +138,20 @@ private fun ConeTypeParameterType.hasNotNullUpperBound(): Boolean {
         }
     }
 }
+
+val FirTypeRef.canBeNull: Boolean
+    get() = coneType.canBeNull
+
+val ConeKotlinType.canBeNull: Boolean
+    get() {
+        if (isMarkedNullable) {
+            return true
+        }
+        return when (this) {
+            is ConeFlexibleType -> upperBound.canBeNull
+            is ConeDefinitelyNotNullType -> false
+            is ConeTypeParameterType -> this.lookupTag.typeParameterSymbol.fir.bounds.any { it.canBeNull }
+            is ConeIntersectionType -> intersectedTypes.any { it.canBeNull }
+            else -> isNullable
+        }
+    }