radeonsi: treat nir_intrinsic_load_constant as a VMEM operation

radeonsi: treat nir_intrinsic_load_constant as a VMEM operation

This is used by variable indexing of constant arrays, to build
code like this:

   s_add_u32 s6, s6, const_data@rel32@lo+4
   s_addc_u32 s7, s7, const_data@rel32@hi+12
   [...]
   global_load_dword v4, v4, s[6:7

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/5118
Fixes: 8288882965b ("radeonsi: set MEM_ORDERED optimally")
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13550>