aco: fix determining if LOD is zero for nir_texop_txf/nir_texop_txs

txf/txs expects LOD to be a 32-bit unsigned integer while other
texture operations expects a float.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/3668
Fixes: 93c8ebfa780 ("aco: Initial commit of independent AMD compiler")
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7256>