FIR: Add nullability smartcast after !is check

^KT-39072 Fixed

compiler/fir/analysis-tests/testData/resolve/smartcasts/safeCalls/boundSafeCallAndIsCheck.kt

@@ -0,0 +1,17 @@
+interface A {
+    val b: B
+}
+
+interface B
+interface C : B {
+    fun q(): Boolean
+}
+
+fun A.foo(): String = ""
+
+fun main(a: A?) {
+    val lb = a?.b
+    if (lb !is C) return
+
+    a.foo().length
+}