aco: move attempt to find strided register into get_reg_simple()

This simplifies code and helps some shaders

Totals from affected shaders:
Code Size: 51227172 -> 51202216 (-0.05 %) bytes
Max Waves: 19955 -> 19948 (-0.04 %)

Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4573>
1 file changed