nir: Implement a Mesa IR -> NIR translator.

nir: Implement a Mesa IR -> NIR translator.

Shamelessly ripped off from Eric Anholt's tgsi_to_nir pass.

This is not built on SCons, like the rest of NIR.

v2:
- Delete redundant c->s, c->impl, and c->cf_node_list pointers (Ken)
- Use nir_builder directly instead of ptn_compile in more places (Ken)
- Drop 'struct' keyword in front of nir_builder (ken)
- Add a file level Doxygen comment (Ken)
- Use scalar constants instead of splatting (Eric)
- Use nir_builder helpers for constants, moves, and swizzles (Connor)

v3: Minor indentation improvements.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>