Omit asserts if condition can be proven statically

[dcodeIO]

As mentioned in #681 (comment), this PR makes assert try to prove an assertion statically, omitting generating any code if possible.

Shrinks some tests significantly as a side-product. Not 100% sure if it's sufficient this way, or whether a static_assert builtin would be a better choice.

[MaxGraey]

May be better skip this optimization when using --debug or -O0?

[dcodeIO]

Hmm, when I think about it, it's actually very similar to static branch elimination just that the branch here is an abort. It shouldn't generate any useful debug information if not hit anyway, except that there's an abort in the code that never executes referencing a (non-human-readable) string offset in memory. Hmm, hmm...

[dcodeIO]

Going to merge this to get it out of the way, but let's all keep an eye open for potential downsides to this. If it doesn't work out well, it should be easy to reverse.