r300: Fix fragment program instruction pairing and register allocation

r300: Fix fragment program instruction pairing and register allocation

There were a number of bugs related to the pairing of vector and scalar
operations where swizzles ended up using the wrong source register,
or an instruction was moved forward and ended up overwriting an aliased
register.

The new algorithm for register allocation is quite conservative and may
run out of registers before necessary. On the plus side, It Just Works.

Pairing is done whenever possible, and in more cases than before, so
in practice this change should be a net win.