nir: introduce new nir_alu_alu_width() with nir_vectorize_cb callback

This function allows to only scalarize instructions down to a desired
vectorization width. nir_lower_alu_to_scalar() was changed to use the
new function with a width of 1.

Swizzles outside vectorization width are considered and reduce
the target width.

This prevents ending up with code like
  vec2 16 ssa_2 = iadd ssa_0.xz, ssa_1.xz

which requires to emit shuffle code in backends
and usually is not beneficial.

Reviewed-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13080>

src/compiler/nir/meson.build

@@ -128,7 +128,7 @@ files_libnir = files(
   'nir_loop_analyze.c',
   'nir_loop_analyze.h',
   'nir_lower_alu.c',
-  'nir_lower_alu_to_scalar.c',
+  'nir_lower_alu_width.c',
   'nir_lower_alpha_test.c',
   'nir_lower_amul.c',
   'nir_lower_array_deref_of_vec.c',