In order to set up KILL sets, the dataflow code was walking the entire
array of ACPs for every instruction.  If you assume the number of ACPs
increases roughly with the number of instructions, this is O(n^2).  As
it turns out, regions_overlap() is not nearly as cheap as one would like
and shows up as a significant chunk on perf traces.

This commit changes things around and instead first builds an array of
exec_lists which it uses like a hash table (keyed off ACP source or
destination) similar to what's done in the rest of the copy-prop code.
By first walking the list of ACPs and populating the table and then
walking instructions and only looking at ACPs which probably have the
same VGRF number, we can reduce the complexity to O(n).  This takes the
execution time of the piglit vs-isnan-dvec test from about 56.4 seconds
on an unoptimized debug build (what we run in CI) with NIR_VALIDATE=0 to
about 38.7 seconds.

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