aco: Fix uniform i2i64.

Fixes 240 failing test cases in dEQP-VK.spirv_assembly which
were failing due to a bad s_ashr_i32 instruction. This commit
fixes the instruction format along with the definitions of the
instruction.

Fixes: 11f43caa
Cc: 19.3 <mesa-stable@lists.freedesktop.org>
Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
(cherry picked from commit 11e62a97)