INVARIANTS sometimes informally documents some of the less obvious
assumptions that trip up clang --analyze with a path into a __dead2
function. Turning it on doesn't affect much else, so let's do it.
This is going to hide a host of write-only or straight up unused vars when the kernel is built without invariants.
If anything the thing to do here is to rework all the assert macros to expand to __builtin_unreachable if the asserted condition does not hold. However, that's quite a churn.