nir: Drop nir_alu_dest

Instead, we replace it directly with nir_def.  We could replace it with
nir_dest but the next commit gets rid of that so this avoids unnecessary
churn.  Most of this commit was generated by sed:

   sed -i -e 's/dest.dest.ssa/def/g' src/**/*.h src/**/*.c src/**/*.cpp

There were a few manual fixups required in the nir_legacy.c and
nir_from_ssa.c as nir_legacy_reg and nir_parallel_copy_entry both have a
similar pattern.

Acked-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24674>

docs/nir/alu.rst

@@ -65,9 +65,6 @@ use in other instruction types like intrinsics or texture ops.
 .. doxygenstruct:: nir_alu_src
    :members:
 
-.. doxygenstruct:: nir_alu_dest
-   :members:
-
 NIR ALU Opcode Reference:
 -------------------------