aco: Eliminate useless exec writes in jump threading.
Eliminate exec writes which are unused by subsequent instructions. Fossil DB results on Sienna Cichlid: Totals from 80960 (54.03% of 149839) affected shaders: CodeSize: 162953748 -> 161749372 (-0.74%) Instrs: 31462273 -> 31161179 (-0.96%) Copies: 2171239 -> 1942293 (-10.54%) Branches: 807771 -> 807747 (-0.00%) Signed-off-by:Timur Kristóf <timur.kristof@gmail.com> Reviewed-by:
Tony Wasserka <tony.wasserka@gmx.de> Part-of: <mesa/mesa!10691>