aco: Eliminate useless exec writes in jump threading.

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: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10691>