intel/fs: Add support for CS to group invocations in quads

intel/fs: Add support for CS to group invocations in quads

When using quads, instead of mapping the elements to the next 4 local
invocation indices, we map the two next in the "current" row and two
next in the "next row".  A side effect is that a thread will execute
the indices in a different order.

We now perform the lowering of both local invocation ID and index
together -- and don't rely anymore on lowering done by
nir_lower_system_values.  That is convenient when doing the math for
quads, because we need X and Y to get the right invocation index.

When the pass progresses, fold the constants and clean up to reduce
the noise from the indexing math.

This implements the derivative_group_quadsNV semantics from
NV_compute_shader_derivatives.

v2: Take subgroup_id into account, otherwise only values in the first
    subgroup would be used. (Jason)

v3: Calculate invocation index and ID together, to avoid duplicating
    some math in the quads case when both index and ID are used. (Jason)

v4: Don't call cleanup passes as part of the lowering, let that to the
    call site. (Jason)
    Change calculation to use less instructions. (Jason)

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com> (v3)
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>