i965/nir/vec4: Implement load_const intrinsic

i965/nir/vec4: Implement load_const intrinsic

Similar to fs_nir backend, a nir_local_values map will be filled with
newly allocated registers as the load_const instrinsic instructions are
processed. Later, get_nir_src() will fetch the registers from this map
for sources that are ssa.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>