llvmpipe: fix nir dot products (fsum op)

When the dot product uses a source which can be optimized to a scalar, after a bunch of nir optimization steps the source to fsum will be a scalar with a x replicate swizzle. Hence nir_src_num_components is just 1 and the fsum was just a no-op which is not correct. Arguably this could be optimized a bit better, but just determine the number of addends by using nir_op_infos instead (the operand fetch was fixed already by 39a938ec doing the same).