[BUG] Codegen testing

[DenisYaroshevskiy]

At the moment all of the testing is about correctness.
However, the many if constexpr branches can be not written correctly (and we saw a few examples like that).

We need codegen testing: smth a long the lines of: this function will produce the following instruction.
We discussed that one compiler should be enough and it's hard to do more than that, since the codegen will vary between the compilers.

[DenisYaroshevskiy]

One example: https://github.com/jfalcou/eve/pull/394/files

[DenisYaroshevskiy]

PR: #413