generate !! if the postfix operator is nullable

tests/testData/ast/issues/file/kt-824-isDir.kt

@@ -3,7 +3,7 @@ import java.io.File
 public open class Test() {
 class object {
 public open fun isDir(parent : File?) : Boolean {
-if (parent == null || !parent?.exists())
+if (parent == null || !parent?.exists()!!)
 {
 return false
 }

tests/testData/ast/prefixOperator/statement/nullableIf.jav

@@ -0,0 +1,2 @@
+String s = null;
+if (!s.isEmpty()) { }

tests/testData/ast/prefixOperator/statement/nullableIf.kt

@@ -0,0 +1,4 @@
+var s : String? = null
+if (!s?.isEmpty()!!)
+{
+}