Dead code elimination

#KT-6602 Fixed
 #KT-6305 Fixed
 #KT-5656 Fixed

compiler/testData/codegen/bytecodeText/deadCodeElimination/boxing.kt

@@ -0,0 +1,16 @@
+fun foo(x: Any?) {}
+
+fun box() {
+    val x: Int? = 1
+    x!!
+
+    val z: Int? = if (1 == 1) x else null
+    z!!
+
+    foo(1 as java.lang.Integer)
+}
+
+// 0 IFNULL
+// 1 IFNONNULL
+// 1 throwNpe
+// 0 ATHROW