agx: Handle 8-bit vecs

These should "just" work, promoting the 8-bit channels to 16-bit registers
internally, allowing us to use our 8-bit stores with 8-bit data vectors packed
in 16-bit registers. All other non-conversion ALU gets lowered by the previous
patch, this is just needed for simple things like nir_op_vec of lowered math
passed to a vectorized store.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24635>

src/asahi/compiler/agx_compile.c

@@ -1202,7 +1202,8 @@ agx_emit_alu(agx_builder *b, nir_alu_instr *instr)
    ASSERTED unsigned comps = nir_dest_num_components(instr->dest.dest);
 
    assert(comps == 1 || nir_op_is_vec(instr->op));
-   assert(sz == 1 || sz == 16 || sz == 32 || sz == 64);
+   assert(sz == 1 || (nir_op_is_vec(instr->op) && sz == 8) || sz == 16 ||
+          sz == 32 || sz == 64);
 
    agx_index dst = agx_dest_index(&instr->dest.dest);
    agx_index s0 = srcs > 0 ? agx_alu_src_index(b, instr->src[0]) : agx_null();