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/testData/diagnostics/tests/j+k/flexibleNothing.kt

@@ -0,0 +1,17 @@
+// FILE: TestClass.java
+import org.jetbrains.annotations.Nullable;
+public class TestClass {
+    public <T> T set(@Nullable String key, @Nullable T t) {
+        return t;
+    }
+}
+
+// FILE: main.kt
+fun run() {
+    val testClass = TestClass()
+    // inferred as `set<Nothing>()`, return type is Nothing!
+    testClass.set("test", null)
+
+    // Should not be unreachable
+    run()
+}