nvk: Make texture descriptors a bit more acceptable to codegen

nvk: Make texture descriptors a bit more acceptable to codegen

Codegen assumes that it gets a combined descriptor on both the texture
and sampler handle sources and that it's 64-bit.  It's not too hard to
just give it that in NIR.  We may make codegen a bit more flexible in
future but, for now, this gets things working.  While we're here, do a
quick optimization for combined texture+sampler descriptors because it's
easy.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24326>