radeonsi: enable ACO by default

NIR+ACO is the best SSA-based shader compiler for AMD GPUs that exists.

There are many reasons why NIR+ACO is better than LLVM, and I have a long
list that I've collected over the years, but the major ones are better GPU
performance (faster GPU memory access thanks to better clauses and
scheduling, a lot less SGPR/VGPR spilling, better loop support, slightly
smaller shader binaries), 8x lower shader compile times, and smaller memory
footprint of the IR.

It also shows that NIR is a mature SSA-based shader compiler that helps
drivers generate optimized code very quickly.

And most importantly, radeonsi has slightly better Viewperf performance
with NIR+ACO than LLVM, and that's difficult to ignore.

Reviewed-by: Qiang Yu <yuq825@gmail.com>
Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/38070>

docs/envvars.rst

@@ -1784,8 +1784,8 @@ RadeonSI driver environment variables
       Use old-style monolithic shaders compiled on demand
    ``nooptvariant``
       Disable compiling optimized shader variants.
-   ``useaco``
-      Use ACO as shader compiler when possible
+   ``usellvm``
+      Use LLVM as shader compiler when possible
    ``nowc``
       Disable GTT write combining
    ``check_vm``