nir: Add an array splitting pass

nir: Add an array splitting pass

This pass looks for array variables where at least one level of the
array is never indirected and splits it into multiple smaller variables.

This pass doesn't really do much now because nir_lower_vars_to_ssa can
already see through arrays of arrays and can detect indirects on just
one level or even see that arr[i][0][5] does not alias arr[i][1][j].
This pass exists to help other passes more easily see through arrays of
arrays.  If a back-end does implement arrays using scratch or indirects
on registers, having more smaller arrays is likely to have better memory
efficiency.

v2 (Jason Ekstrand):
 - Better comments and naming (some from Caio)
 - Rework to use one hash map instead of two

v2.1 (Jason Ekstrand):
 - Fix a couple of bugs that were added in the rework including one
   which basically prevented it from running

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>