ir3: make shpe a terminator

shpe is a bit of a special instruction: it's not really a terminator
(i.e., it does not perform a jump) but it does have to stay at the end
of its block. Up to now, we tried to enforce this by creating const
write barriers on shpe; the assumption being that everything that
happens in the preamble ends in a write to the const file so shpe stays
at the end. Alas, it turns out this is not true: things like sampler
prefetches do not write the const file and nothing was preventing those
from being scheduled after shpe.

Instead of trying to create even more barrier dependencies, fix this by
making shpe a terminator. Both sched and postsched treat terminators
specially to make sure they always stay at the end of their block.

Signed-off-by: Job Noorman <jnoorman@igalia.com>
Part-of: <!34290>