intel/nir: Split IO arrays into elements

intel/nir: Split IO arrays into elements

The NIR nir_lower_io_arrays_to_elements pass attempts to split I/O
variables which are arrays or matrices into a sequence of separate
variables.  This can help link-time optimization by allowing us to
remove varyings at a more granular level.

Shader-db results on Kaby Lake:

    total instructions in shared programs: 15177645 -> 15168494 (-0.06%)
    instructions in affected programs: 79857 -> 70706 (-11.46%)
    helped: 392
    HURT: 0

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>