mesa/st: Only call nir_lower_io_to_scalar_early on scalar ISAs

mesa/st: Only call nir_lower_io_to_scalar_early on scalar ISAs

On scalar ISAs, nir_lower_io_to_scalar_early enables significant
optimizations. However, on vector ISAs, it is counterproductive and
impedes optimal codegen. This patch only calls
nir_lower_io_to_scalar_early for scalar ISAs. It appears that at present
there are no upstreamed drivers using Gallium, NIR, and a vector ISA, so
for existing code, this should be a no-op. However, this patch is
necessary for the upcoming Panfrost (Midgard) and Lima (Utgard)
compilers, which are vector.

With this patch, Panfrost is able to consume NIR directly, rather than
TGSI with the TGSI->NIR conversion.

For how this affects Lima, see
https://www.mail-archive.com/mesa-dev@lists.freedesktop.org/msg189216.html

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>