anv/gen12: Lower VK_KHR_multiview using Primitive Replication

anv/gen12: Lower VK_KHR_multiview using Primitive Replication

Identify if view_index is used only for position calculation, and use
Primitive Replication to implement Multiview in Gen12.  This feature
allows storing per-view position information in a single execution of
the shader, treating position as an array.

The shader is transformed by adding a for-loop around it, that have an
iteration per active view (in the view_mask).  Stores to the position
now store into the position array for the current index in the loop,
and load_view_index() will return the view index corresponding to the
current index in the loop.

The feature is controlled by setting the environment variable
ANV_PRIMITIVE_REPLICATION_MAX_VIEWS, which defaults to 2 if unset.
For pipelines with view counts larger than that, the regular
instancing will be used instead of Primitive Replication.  To disable
it completely set the variable to 0.

v2: Don't assume position is set in vertex shader; remove only stores
    for position; don't apply optimizations since other passes will
    do; clone shader body without extract/reinsert; don't use
    last_block (potentially stale). (Jason)

    Fix view_index immediate to contain the view index, not its order.
    Check for maximum number of views supported.
    Add guard for gen12.

v3: Clone the entire shader function and change it before reinsert;
    disable optimization when shader has memory writes. (Jason)

    Use a single environment variable with _DEBUG on the name.

v4: Change to use new nir_deref_instr.
    When removing stores, look for mode nir_var_shader_out instead
    of the walking the list of outputs.
    Ensure unused derefs are removed in the non-position part of the
    shader.
    Remove dead control flow when identifying if can use or not
    primitive replication.

v5: Consider all the active shaders (including fragment) when deciding
    that Primitive Replication can be used.
    Change environment variable to ANV_PRIMITIVE_REPLICATION.
    Squash the emission of 3DSTATE_PRIMITIVE_REPLICATION into this patch.
    Disable Prim Rep in blorp_exec_3d.

v6: Use a loop around the shader, instead of manually unrolling, since
    the regular unroll pass will kick in.
    Document that we don't expect to see copy_deref or load_deref
    involving the position variable.
    Recover use_primitive_replication value when loading pipeline from
    the cache.
    Set VARYING_SLOT_LAYER to 0 in the shader.  Earlier versions were
    relying on ForceZeroRTAIndexEnable but that might not be
    sufficient.
    Disable Prim Rep in cmd_buffer_so_memcpy.

v7: Don't use Primitive Replication if position is not set, fallback
    to instancing; change environment variable to be
    ANV_PRIMITVE_REPLICATION_MAX_VIEWS and default it to 2 based on
    experiments.

Reviewed-by: Rafael Antognolli <rafael.antognolli@intel.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/2313>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/2313>