ac/nir: clear unused components before storing XFB outputs to LDS

ac/nir: clear unused components before storing XFB outputs to LDS

Shader variables don't always exactly match intrinsics and they might
contain unused slots.

Fixes a bunch of regressions with RADV_PERFTEST=ngg_streamout on RDNA2,
and also fixes RDNA3 NGG streamout.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/8099
Fixes: cd22bf90e79 ("ac/nir/ngg: refine nogs outputs handling")
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20735>