nir: Add a large constants optimization pass

nir: Add a large constants optimization pass

This pass searches for reasonably large local variables which can be
statically proven to be constant and moves them into shader constant
data.  This is especially useful when large tables are baked into the
shader source code because they can be moved into a UBO by the driver to
reduce register pressure and make indirect access cheaper.

v2 (Jason Ekstrand):
 - Use a size/align function to ensure we get the right alignments
 - Use the newly added deref offset helpers

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>