radv: Revert divisor = 0 case for vertex attribute extension.

Seems like DXVK depends on that and it might get reverted
upstream. Since apps are not supposed to use 0 in v2 anyway,
we should be safe implementing the old behavior there.

Fixes: 66e12451ac "radv: Update to new VK_EXT_vertex_attribute_divisor to version 2."
CC: 18.2 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>

src/amd/vulkan/radv_nir_to_llvm.c

@@ -2006,7 +2006,7 @@ handle_vs_input_decl(struct radv_shader_context *ctx,
 						MAX2(1, ctx->shader_info->vs.vgpr_comp_cnt);
 				}
 			} else {
-				unreachable("Invalid vertex attribute divisor of 0.");
+				buffer_index = ctx->ac.i32_0;
 			}
 
 			buffer_index = LLVMBuildAdd(ctx->ac.builder, ctx->abi.start_instance, buffer_index, "");