Support lateinit local vars in redundant null check elimination

Lateinit local vars are guaranteed to be non-null after store.
So we mark such stores as storing non-null value
(could be useful for some other constructs, too),
and optimize null checks accordingly.

compiler/testData/codegen/bytecodeText/nullCheckOptimization/localLateinit/initialized.kt

@@ -0,0 +1,12 @@
+// LANGUAGE_VERSION: 1.2
+
+fun test() {
+    lateinit var z: String
+    run {
+        z = ""
+    }
+    println(z)
+}
+
+// 0 IFNULL
+// 0 IFNONNULL