nir: Add a pass for lowering IO back to vector when possible

This pass tries to turn scalar and array-of-scalar IO variables into
vector IO variables whenever possible.

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>
Cc: "19.0" <mesa-stable@lists.freedesktop.org>

src/compiler/nir/meson.build

@@ -133,6 +133,7 @@ files_libnir = files(
   'nir_lower_io_arrays_to_elements.c',
   'nir_lower_io_to_temporaries.c',
   'nir_lower_io_to_scalar.c',
+  'nir_lower_io_to_vector.c',
   'nir_lower_packing.c',
   'nir_lower_passthrough_edgeflags.c',
   'nir_lower_patch_vertices.c',