nir: Add a cubemap normalizing pass

nir: Add a cubemap normalizing pass

This commit adds a pass to L1-normalize cube-map coordinates.  Some hardware
such as i965 requires that largest cube-map coordinate is +-1.  We had a
pass to perform this normalization in GLSL IR but we need it in NIR for
cube maps on ARB programs to work correctly.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
v2 (Suggested by Eric):
 - Do a vector fabs and split into components later
 - Move to core NIR

Reviewed-by: Eric Anholt <eric@anholt.net>