nir: Introduce a nir_vec_scalars() helper using nir_ssa_scalar.

nir: Introduce a nir_vec_scalars() helper using nir_ssa_scalar.

Many users of nir_vec() do so by nir_channel()-ing a new ssa defs as movs
from other vectors to put the new vector together, which then just have to
get copy-propagated into the ALU srcs and DCEed away the temporary movs.
If they instead take nir_ssa_scalar, we can avoid that extra work.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14865>