llvmpipe: implement dual source blending

llvmpipe: implement dual source blending

link up the fs outputs and blend inputs, and make sure the second blend source
is correctly loaded and converted (which is quite complex).
There's a slight refactoring of the monster generate_unswizzled_blend()
function where it makes sense to factor out alpha conversion (which needs
to run twice for dual source blend).
This passes piglit arb_blend_func_extended tests.

v2: remove new but ultimately not used function...

Reviewed-by: Brian Paul <brianp@vmware.com>