Do a proper compatibility/capability check before generating backend code

[ohm314]

All checks we are doing now before emitting ISPC code are rather fragile. When redoing the code generation framework we should take care that for the various backends (ISPC, cuda, openacc, ...) there is a robust way to check if a compute routine (and dependent code) can be emitted for that target or it has to fallback.

[alkino]

Should be close with #962 and #963?