Make expr simplifier tests more readable

[zasdfgbnm]

I am really tired of reading and writing tests for expr simplifier, because things like

    auto val =
        mul(mul(add(add(mul(c, d), add(add(e, f), three)), three),
                add(add(add(add(a, b), three), five), c)),
            a);

isn't really human-readable.

So, I write a tiny compiler to compile strings like

( ( ( ( ( i2 * i3 ) + ( ( i4 + i5 ) + 3 ) ) + 3 ) * ( ( ( ( i0 + i1 ) + 3 ) + 5 ) + i2 ) ) * i0 )

into Val*.

We can just copy-paste from the generated C++ code, or inline-printed Val*, and this compiler should be able to compile it.

Using this simple compiler, I can replace most of test_expr_simplifier.cpp with a more readable format.

[zasdfgbnm]

I suggest reviewing this file while hiding white space.

[zasdfgbnm]

This is a bug fix; see description. This issue does not only happen on unit tests, it does happen on real fusions.

[naoyam]

Cool! I haven't completely checked the "stupid" compiler, but if it passes these tests, but approves as the tests seem comprehensive.