[K/N] Rework is checks and as casts codegeneration

^KT-58707
^KT-59022

compiler/testData/codegen/box/casts/kt50577.kt

@@ -1,5 +1,3 @@
-// IGNORE_BACKEND: NATIVE
-
 abstract class A {
     abstract val x: Any
 

compiler/testData/codegen/box/casts/kt58707.kt

@@ -0,0 +1,7 @@
+interface A {}
+
+fun <E> getA() = (object : A {}) as A
+
+fun box() : String {
+    return if (getA<Int>() is A) "OK" else "FAIL"
+}

compiler/testData/codegen/box/casts/kt59022.kt

@@ -0,0 +1,20 @@
+// IGNORE_BACKEND: JS, JS_IR, JS_IR_ES6, WASM
+@file:Suppress("INCOMPATIBLE_TYPES", "UNCHECKED_CAST")
+
+fun <T> unchecked(x: Any?) = x as T
+
+fun box() : String{
+    if (unchecked<String>(null) is Any) return "FAIL 1"
+    if (unchecked<String>(null) is IntArray) return "FAIL 2"
+    try {
+        unchecked<String>(null) as Any
+        return "FAIL 3"
+    } catch (e: NullPointerException) {
+    }
+    try {
+        unchecked<String>(null) as IntArray
+        return "FAIL 4"
+    } catch (e: NullPointerException) {
+    }
+    return "OK"
+}