Fix false positive unreachable code in case of Nothing!-typed calls

Note, that this change potentially has some other effects in corner cases
(like the changed test data that is rather sensible because `bar`
in the example is not effectively projected out and can be called
with nulls)

Probably, we need to consider rewriting all other isSomeType methods
in KotlinBuiltins, but now it seems to be a rather dangerous change

 #KT-16424 Fixed

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

@@ -12578,6 +12578,12 @@ public class DiagnosticsTestGenerated extends AbstractDiagnosticsTest {
                 doTest(fileName);
             }
 
+            @TestMetadata("flexibleNothing.kt")
+            public void testFlexibleNothing() throws Exception {
+                String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/j+k/flexibleNothing.kt");
+                doTest(fileName);
+            }
+
             @TestMetadata("genericConstructorWithMultipleBounds.kt")
             public void testGenericConstructorWithMultipleBounds() throws Exception {
                 String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/j+k/genericConstructorWithMultipleBounds.kt");