[NI] Improve CST algorithm to handle non-fixed variables

#KT-32456 Fixed
 #KT-32423 Fixed
 #KT-32818 Fixed
 #KT-33197 Fixed

compiler/testData/codegen/box/elvis/genericElvisWithMoreSpecificLHS.kt

@@ -0,0 +1,18 @@
+// !LANGUAGE: +NewInference
+// WITH_RUNTIME
+// IGNORE_BACKEND: JS_IR
+
+fun test(foo: MutableList<String>?): List<String> {
+    val bar = foo ?: listOf()
+    return bar
+}
+
+fun box(): String {
+    val a = test(null)
+    if (a.isNotEmpty()) return "Fail 1"
+
+    val b = test(mutableListOf("a"))
+    if (b.size != 1) return "Fail 2"
+
+    return "OK"
+}

compiler/testData/codegen/box/elvis/genericElvisWithNullLHS.kt

@@ -0,0 +1,23 @@
+// WITH_RUNTIME
+// IGNORE_BACKEND: JS_IR
+
+fun test() {
+    fun returnMutableList(): MutableList<Int>? = null
+    fun returnsList(): List<Int>? = null
+
+    var mutableList: MutableList<Int>? = null
+    var list: List<Int>? = null
+
+    mutableListOf<Int>().addAll(returnMutableList() ?: emptyList<Int>())
+    mutableListOf<Int>().addAll(returnsList() ?: emptyList())
+    mutableListOf<Int>().addAll(list ?: emptyList())
+
+    mutableListOf<Int>().addAll(returnMutableList() ?: emptyList())
+    mutableListOf<Int>().addAll(mutableList ?: emptyList())
+    mutableListOf<Int>().addAll(null ?: emptyList())
+}
+
+fun box(): String {
+    test()
+    return "OK"
+}