r300/compiler: Refactor the pair instruction data structures

r300/compiler: Refactor the pair instruction data structures

Use rc_pair_ prefix for all pair instruction structs

Create a named struct for pair instruction args

Replace structs radeon_pair_instruction_{rgb,alpha} with struct
radeon_pair_sub_instruction.  These two structs were nearly identical
and were creating a lot of cut and paste code.  These changes are the
first step towards removing some of that code.