libkern: Make KASSERT verify expression is valid if !DIAGNOSTIC.

This way it is no longer necessary to mark variables __diagused if
they are used in KASSERT conditions.

Fix fallout from this by removing now-unnecessary and `#ifdef
DIAGNOSTIC'.

Don't do the same for KDASSERT if !DEBUG -- unlike KASSERT and
DIAGNOSTIC, variables needed by KDASSERT and DEBUG are likely to be
expensive to compute (and potentially difficult for a compiler to
prove flushable), so we don't want to require them under !DEBUG.

(riastradh)