nir: Add fsat_signed, fclamp_pos opcodes

These correspond to destination modifiers on Mali GPUs, and this allows us to use algebraic ops to fuse them. (Eventually, we'll treat them as destination modifiers via to-be-polished helpers.) fsat_signed in particular might be convenient for format conversion as well (the pattern is open-coded for SNORM); if so, we could add a shader compiler option for lowering in the future if that's interesting.