gallivm: checkpoint: nearest mipmap filtering

gallivm: checkpoint: nearest mipmap filtering

The LOD is computed from texcoord partial derivatives and used to
select a mipmap level.  Still some bugs in texel fetching.  Lots of
rough edges and unfinished parts but the basics are in place.

Lots of changes to the lp_bld_arit.c code to support non-vector/scalar
datatypes.