nir: Separate dead write removal into its own pass

nir: Separate dead write removal into its own pass

Instead of doing this as part of the existing copy_prop_vars pass.

Separation makes easier to expand the scope of both passes to be more
than per-block.  For copy propagation, the information about valid
copies comes from previous instructions; while the dead write removal
depends on information from later instructions ("have any instruction
used this deref before overwrite it?").

Also change the tests to use this pass (instead of copy prop vars).
Note that the disabled tests continue to fail, since the standalone
pass is still per-block.

v2: Remove entries from dynarray instead of marking items as
    deleted.  Use foreach_reverse. (Caio)

    (all from Jason)
    Do not cache nir_deref_path.  Not worthy for this patch.
    Clear unused writes when hitting a call instruction.
    Clean up enumeration of modes for barriers.
    Move metadata calls to the inner function.

v3: For copies, use the vector length to calculate the mask.

    (all from Jason)
    Use nir_component_mask_t when applicable.
    Rename functions for clarity.
    Consider local vars used by a call to be conservative (SPIR-V has
    such cases).
    Comment and assert the assumption that stores and copies are
    always to a deref that ends with a vector or scalar.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>