ddebug: handle some cases of non-TGSI shaders

ddebug: handle some cases of non-TGSI shaders

NIR shaders are not captured properly in pipelined mode currently. This
would require shader cloning, which requires linking all the Gallium
drivers against NIR. We can always do that later.

v2: avoid immediate crashes in pipelined mode

Reviewed-by: Marek Olšák <marek.olsak@amd.com> (v1)