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>

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