zink: add lengthy comment and remove assert from discard_if ntv pass

zink: add lengthy comment and remove assert from discard_if ntv pass

as in the comment, while we may want to try verifying that discard will be
the last instruction in a block, it's a bit problematic given that other nir
passes we're doing may insert instructions after a discard as part of e.g.,
nir_opt_dead_cf in the process of removing another block

fixes shaders@glsl-fs-discard-04

Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5852>