i965/nir/vec4: Add implementation placeholders for a new NIR->vec4 pass

i965/nir/vec4: Add implementation placeholders for a new NIR->vec4 pass

This patch will add a brw_vec4_nir.cpp file filled with entry point methods to
the main functionality, following a structure similar to brw_fs_nir.cpp.

Subsequent patches in this series will be adding the implementations for these
methods, incrementally.

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