nir: Add load_typed_buffer_amd intrinsic.

nir: Add load_typed_buffer_amd intrinsic.

This new intrinsic maps to the MTBUF instruction format on AMD GPUs
and represents a typed buffer load in NIR.

Also add an unsigned upper bound for the new intrinsic.
Code for that ported from aco_instruction_selection_setup.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Konstantin Seurer <konstantin.seurer@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16805>