debug checks for lqref usage

[sgraham]

lqrefs are references into the current Fn, this can be error prone as it's easy to try to cache them across lq_func_start/ends.

one possible solution would be to use a few bits at the top of lqref.u as a generation counter (for each lq_func_start). even only 4 bits would probably catch almost all accidental misuses.