r500_fragprog: Major refactoring of final emit

r500_fragprog: Major refactoring of final emit

Use an abstracted instruction scheduling and register allocation algorithm
that we will be able to share with r300_fragprog.

Unlike the original emit code, this code tries to pair instructions that
only use the RGB part of the ALU with instructions that only use the alpha
part. However, the pairing algorithm still has some shortcomings;
for example, it doesn't generate optimal code for the emulation of LIT.