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/ifUnitEqualsNull.kt

@@ -0,0 +1,18 @@
+// WITH_RUNTIME
+
+fun test1() {
+    val a = Unit
+
+    if (a != null) {
+        println("X1")
+    }
+
+    if (a == null) {
+        println("X2")
+    }
+}
+
+// 0 IFNULL
+// 0 IFNONNULL
+// 1 X1
+// 0 X2