nv50/ir: Convert to new-style NIR registers

nv50/ir: Convert to new-style NIR registers

Shader-db results on Turing:

    total inst in shared programs : 11121531 -> 11121458 (-0.00%)
    total gpr in shared programs : 1848287 -> 1848425 (0.01%)
    total ugpr in shared programs : 0 -> 0 (0.00%)
    total local in shared programs : 27200 -> 27200 (0.00%)
    total shared in shared programs : 236476 -> 236476 (0.00%)
    total bytes in shared programs : 177944496 -> 177943328 (-0.00%)
    total cached in shared programs : 0 -> 0 (0.00%)

               inst      gpr     ugpr    local   shared    bytes   cached
    helped      470       50        0        0        0      470        0
      hurt      327      197        0        0        0      327        0

Reviewed-by: Karol Herbst <kherbst@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24110>