[FIR] Fix collecting member candidates on receiver with smartcast

^KT-51460 Fixed
^KT-51827

compiler/testData/diagnostics/tests/smartCasts/kt51460.kt

@@ -0,0 +1,18 @@
+// FIR_IDENTICAL
+// ISSUE: KT-51460
+
+abstract class A {
+    abstract protected val a: A?
+
+    class B(override val a: A?) : A() {
+        fun f(other: A) {
+            val x = if (other is C) {
+                other.a
+            } else {
+                null
+            }
+        }
+    }
+
+    class C(override val a: A?): A()
+}