ir3: Use stp/ldp base offset for {load,store}_scratch

ir3: Use stp/ldp base offset for {load,store}_scratch

When we have a series of loads/stores we were creating a constant for
each one, which isn't great. Furthermore, because the nir pass puts the
offset constant at the top of the shader, it resulted in extra register
pressure and spilling when that happened inside a loop. Fix this by
using the base/offset form of stp and ldp.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13307>