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

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

@@ -0,0 +1,7 @@
+fun foo() {
+    return
+    // val xyz has empty live range because everything after return will be removed as dead
+    val xyz = 1
+}
+
+// 0 xyz

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

@@ -0,0 +1,23 @@
+import kotlin.test.assertEquals
+
+fun foo(value: Boolean): Int {
+    if (value)
+        return 1
+    else
+        return 2
+}
+
+fun box(): String {
+    assertEquals(1, foo(true))
+    assertEquals(2, foo(false))
+    
+    return "OK"
+}
+
+/*
+    3 return's are defined in functions
+    2 are from package facade
+    1 is from package clinit
+*/
+
+// 6 RETURN