r600/sfn: Add lowering pass to legalize image access

r600/sfn: Add lowering pass to legalize image access

Make sure only existing images are accessed and that the accessed
coordinates are within the image.
The generated code is quite exponsive, because it encapsulates each
access to an image with two if statements, one to check whether the
image index actually exists (this will get optimized away if the
image selection is direct), and one if statement to check whether
the coordinates are in range. For that reason it will only be enabled
for Cayman were invalid access seems to raise more issues.

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10608>