st/nir: Re-vectorize shader IO
We scalarize IO to enable further optimizations, such as propagating
constant components across shaders, eliminating dead components, and
so on. This patch attempts to re-vectorize those operations after
the varying optimizations are done.
Intel GPUs are a scalar architecture, but IO operations work on whole
vec4's at a time, so we'd prefer to have a single IO load per vector
rather than 4 scalar IO loads. This re-vectorization can help a lot.
Broadcom GPUs, however, really do want scalar IO. radeonsi may want
this, or may want to leave it to LLVM. So, we make a new flag in the
NIR compiler options struct, and key it off of that, allowing drivers
to pick. (It's a bit awkward because we have per-stage settings, but
this is about IO between two stages...but I expect drivers to globally
prefer one way or the other. We can adjust later if needed.)
Reviewed-by: Marek Olšák <marek.olsak@amd.com>