Add test for 'a IS NOT NULL AND a = b' simplification

Simplification (from RexSimplify class) is mostly covered in RexProgramTest, but tests for expressions of the form "IS NOT NULL(a) AND a=b" (with nullable and not nullable types) seem to be missing.
 
Since I had to write tests to make sure the simplification was as expected, I assume others might end up doing the same, and that the tests will both act as documentation and it will also protect against regressions.