freedreno/ir3: enable nir_opt_loop_unroll on a6xx

freedreno/ir3: enable nir_opt_loop_unroll on a6xx

If precision lowering happens at GLSL IR, loop_analysis at IR doesn't
work as expected since it can't handle things like:

"(expression bool < (expression float16_t f2fmp (var_ref ndx) ) (constant float16_t (1.000000)) )"

So we'd rather do this optimization at the NIR stage.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3885>