radv: add support for loading the LSHS vertex stride from a SGPR

radv: add support for loading the LSHS vertex stride from a SGPR

With shader object, if VS and TCS aren't linked together, the LSHS
vertex stride should be computed from the vertex outputs. Otherwise,
if an output is unused, the stride is wrong in TCS.

This is currently for GFX8 only because for merged shaders this won't
be needed but shader object on GFX9+ isn't yet a thing.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24540>