Fix 'equals' for NotNullBasicValue

NotNullBasicValues were merged incorrectly sometimes,
which caused problems with INSTANCEOF checks.

 #KT-18779 Fixed

compiler/testData/codegen/box/regressions/kt18779.kt

@@ -0,0 +1,19 @@
+sealed class Result {
+    class Failure(val exception: Exception) : Result()
+    class Success(val message: String) : Result()
+}
+
+fun box(): String {
+    var result: Result
+    try {
+        result = Result.Success("OK")
+    }
+    catch (e: Exception) {
+        result = Result.Failure(Exception())
+    }
+
+    when (result) {
+        is Result.Failure -> throw result.exception
+        is Result.Success -> return result.message
+    }
+}