Prevent early deleting of not-null assertions

Before val to var conversion applied, smart-cast can cause
dropping not-null assertion, which will be required when val will
become var

j2k/testData/fileOrElement/nullability/LocalValReassignment.java

@@ -0,0 +1,35 @@
+import org.jetbrains.annotations.*;
+
+class A {
+
+    /* rare nullable, handle with caution */
+    public String nullableString() {
+        if (Math.random() > 0.999) {
+            return "a string";
+        }
+        return null;
+    }
+
+    public void takesNotNullString(@NotNull String s) {
+        System.out.println(s.substring(1));
+    }
+
+    public void aVoid() {
+        String aString;
+        if (nullableString() != null) {
+            aString = nullableString();
+            if (aString != null) {
+                for (int i = 0; i < 10; i++) {
+                    takesNotNullString(aString); // Bang-bang here
+                    aString = nullableString();
+                }
+            }
+            else {
+                aString = "aaa";
+            }
+        }
+        else {
+            aString = "bbbb";
+        }
+    }
+}

j2k/testData/fileOrElement/nullability/LocalValReassignment.kt

@@ -0,0 +1,32 @@
+import org.jetbrains.annotations.*
+
+internal class A {
+
+    /* rare nullable, handle with caution */
+    fun nullableString(): String? {
+        return if (Math.random() > 0.999) {
+            "a string"
+        } else null
+    }
+
+    fun takesNotNullString(s: String) {
+        println(s.substring(1))
+    }
+
+    fun aVoid() {
+        var aString: String?
+        if (nullableString() != null) {
+            aString = nullableString()
+            if (aString != null) {
+                for (i in 0..9) {
+                    takesNotNullString(aString!!) // Bang-bang here
+                    aString = nullableString()
+                }
+            } else {
+                aString = "aaa"
+            }
+        } else {
+            aString = "bbbb"
+        }
+    }
+}