[NI] Fix reporting smartcast diagnostics for intersection types

#KT-30826 Fixed

compiler/testData/diagnostics/tests/smartCasts/kt30826.kt

@@ -0,0 +1,19 @@
+// !WITH_NEW_INFERENCE
+// Issue: KT-30826
+
+interface I1
+interface I2 {
+    fun foo() {}
+}
+
+class A : I1, I2
+
+fun foo(x: I1?) {
+    var y = x
+    y as I2
+    val bar = {
+        <!SMARTCAST_IMPOSSIBLE!>y<!>.foo() // NPE in NI, smartcast impossible in OI
+    }
+    y = null
+    bar()
+}