nir: Introduce new nir_intrinsic_load_per_vertex_input intrinsics.

Geometry and tessellation shaders process multiple vertices; their
inputs are arrays indexed by the vertex number.  While GLSL makes
this look like a normal array, it can be very different behind the
scenes.

On Intel hardware, all inputs for a particular vertex are stored
together - as if they were grouped into a single struct.  This means
that consecutive elements of these top-level arrays are not contiguous.
In fact, they may sometimes be in completely disjoint memory segments.

NIR's existing load_input intrinsics are awkward for this case, as they
distill everything down to a single offset.  We'd much rather keep the
vertex ID separate, but build up an offset as normal beyond that.

This patch introduces new nir_intrinsic_load_per_vertex_input
intrinsics to handle this case.  They work like ordinary load_input
intrinsics, but have an extra source (src[0]) which represents the
outermost array index.

v2: Rebase on earlier refactors.
v3: Use ssa defs instead of nir_srcs, rebase on earlier refactors.

Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Jason Ekstrand <[email protected]>

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