ac: split 16-bit ssbo loads that may not be dword aligned

This ends up refactoring visit_load_buffer() a little.

Fixes: 7e7ee826 ('ac: add support for 16bit buffer loads')
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108114


Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>