Add test on smartcast from while-assigned var

Currently behavior is undesired, it will be fixed in the next commit
(see KT-27084)

compiler/testData/diagnostics/tests/smartCasts/loops/whileNullAssignToSomething.kt

@@ -0,0 +1,9 @@
+fun main(args: Array<String>) {
+    var result: String? = null
+    var i = 0
+    while (result == null) {
+        if (i == 10) result = "non null"
+        else i++
+    }
+    result<!UNSAFE_CALL!>.<!>length
+}

compiler/testData/diagnostics/tests/smartCasts/loops/whileNullAssignToSomething.txt

@@ -0,0 +1,3 @@
+package
+
+public fun main(/*0*/ args: kotlin.Array<kotlin.String>): kotlin.Unit

compiler/tests/org/jetbrains/kotlin/checkers/DiagnosticsTestGenerated.java

@@ -19916,6 +19916,11 @@ public class DiagnosticsTestGenerated extends AbstractDiagnosticsTest {
                     runTest("compiler/testData/diagnostics/tests/smartCasts/loops/whileNull.kt");
                 }
 
+                @TestMetadata("whileNullAssignToSomething.kt")
+                public void testWhileNullAssignToSomething() throws Exception {
+                    runTest("compiler/testData/diagnostics/tests/smartCasts/loops/whileNullAssignToSomething.kt");
+                }
+
                 @TestMetadata("whileNullWithBreak.kt")
                 public void testWhileNullWithBreak() throws Exception {
                     runTest("compiler/testData/diagnostics/tests/smartCasts/loops/whileNullWithBreak.kt");

compiler/tests/org/jetbrains/kotlin/checkers/javac/DiagnosticsUsingJavacTestGenerated.java

@@ -19916,6 +19916,11 @@ public class DiagnosticsUsingJavacTestGenerated extends AbstractDiagnosticsUsing
                     runTest("compiler/testData/diagnostics/tests/smartCasts/loops/whileNull.kt");
                 }
 
+                @TestMetadata("whileNullAssignToSomething.kt")
+                public void testWhileNullAssignToSomething() throws Exception {
+                    runTest("compiler/testData/diagnostics/tests/smartCasts/loops/whileNullAssignToSomething.kt");
+                }
+
                 @TestMetadata("whileNullWithBreak.kt")
                 public void testWhileNullWithBreak() throws Exception {
                     runTest("compiler/testData/diagnostics/tests/smartCasts/loops/whileNullWithBreak.kt");