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/ifNullEqualsNullInline.kt

@@ -0,0 +1,19 @@
+// WITH_RUNTIME
+
+// FILE: test.kt
+
+fun test1() {
+    val n = null
+    n.elvis { "X1" }
+    "X2".elvis { "X3" }
+}
+
+// @TestKt.class:
+// 0 IFNULL
+// 0 IFNONNULL
+// 1 X1
+// 1 X2
+// 0 X3
+
+// FILE: inline.kt
+inline fun <T : Any> T?.elvis(rhs: () -> T): T = this ?: rhs()