intel/fs/copy-prop: Purge unused ACPs

intel/fs/copy-prop: Purge unused ACPs

If the destination of an ACP entry exists only within this block, then
there's no need to keep it for dataflow analysis.  We can delete it from
the out_acp table and avoid growing the bitsets any bigger than we
absolutely have to.  This reduces the maximum number of global ACP
entries in the vs-isnan-dvec with software fp64 on Kaby Lake from 8630
to 3942 and takes the execution time of the piglit vs-isnan-dvec test
from about 1:16.2 on an unoptimized debug build (what we run in CI) with
NIR_VALIDATE=0 to about 56.4 seconds.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>