compiler/nir: add a lowering pass to convert the bit size of ALU operations

Not all bit-sizes may be supported natively in hardware for all operations.
This pass allows drivers to lower such operations to a bit-size that is
actually supported and then converts the result back to the original
bit-size.

Compiler backends control which operations and wich bit-sizes require
the lowering through a callback function.

v2: generalize this pass and make it available in NIR core (Rob, Jason)
v3: remove some temporaries and reduce nesting in instruction loop using
    a continue statement (Jason)

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/compiler/nir/meson.build

@@ -141,6 +141,7 @@ files_libnir = files(
   'nir_lower_vec_to_movs.c',
   'nir_lower_wpos_center.c',
   'nir_lower_wpos_ytransform.c',
+  'nir_lower_bit_size.c',
   'nir_metadata.c',
   'nir_move_vec_src_uses_to_dest.c',
   'nir_normalize_cubemap_coords.c',