nir/search: Use larger type to hold linearized index

nir/search: Use larger type to hold linearized index

"index" is an offset into a linearized 3-dimensional array.  Starting
with fbd5359a0a6, the 3-dimensional array can have 43 elements in each
dimension.  43**3 = 79507, and that will overflow the uint16_t.

See also the discussion in MR !3765.

Fixes: fbd5359a0a6 ("nir/algebraic: Rearrange bcsel sequences generated by nir_opt_peephole_select")
Suggested-by: Connor Abbott <cwabbott0@gmail.com>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3871>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3871>