aco: fix imageSize()/textureSize() with large buffers on GFX8

aco: fix imageSize()/textureSize() with large buffers on GFX8

Tested on Navi by using dEQP-VK.image.image_size.buffer.* and the GFX8
path with the size multipled by the stride.
dEQP-VK.image.image_size.buffer.* was also run with the tests modified to
use a 96bit format.

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Fixes: 93c8ebfa780 ('aco: Initial commit of independent AMD compiler')
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3081>