Make redundant null check optimization independent of boxing optimization algorithm.

Run DCE after each single redundant null check optimization pass.

compiler/testData/codegen/bytecodeText/nullCheckOptimization/ifUnitEqualsNullInline.kt

@@ -0,0 +1,18 @@
+// WITH_RUNTIME
+
+// FILE: test.kt
+
+fun test1() {
+    val u = Unit
+    u.mapNullable({ "X1" }, { "X2" })
+}
+
+// @TestKt.class:
+// 0 IFNULL
+// 0 IFNONNULL
+// 1 X1
+// 0 X2
+
+// FILE: inline.kt
+inline fun <T : Any, R> T?.mapNullable(ifNotNull: (T) -> R, ifNull: () -> R) =
+        if (this == null) ifNull() else ifNotNull(this)