nir: Add a pass to lower vector phi nodes to scalar phi nodes

nir: Add a pass to lower vector phi nodes to scalar phi nodes

v2 Jason Ekstrand <jason.ekstrand@intel.com>:
 - Add better comments
 - Use nir_ssa_dest_init and nir_src_for_ssa more places
 - Fix some void * casts

v3 Jason Ekstrand <jason.ekstrand@intel.com>:
 - Rework the way we determine whether or not to sccalarize a phi node to
   make the recursion non-bogus
 - Treat load_const instructions as scalarizable

v4 Jason Ekstrand <jason.ekstrand@intel.com>:
 - Allow uniform and input loads to be scalarizable

v5 Jason Ekstrand <jason.ekstrand@intel.com>:
 - Also consider loads of inputs (varying, uniform, or ubo) to be
   scalarizable.  We were already doing this for load_var on uniforms and
   inputs.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>