nir: fix lower_memcpy

nir: fix lower_memcpy

memcpy is divided into chunks that are vec4 sized max. The problem
here happens with a structure of 24 bytes :

  struct {
    float3 a;
    float3 b;
  }

If you memcpy that struct, the lowering will emit 2 load/store, one of
sized 8, next one sized 16. But both end up located at offset 0, so we
effectively drop 2 floats.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Fixes: a3177cca996145 ("nir: Add a lowering pass to lower memcpy")
Reviewed-by: Jason Ekstrand <jason.ekstrand@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15049>