r300/compiler: Ignore alpha dest register when replicating the result

r300/compiler: Ignore alpha dest register when replicating the result

When the result of the alpha instruction is being replicated to the RGB
destination register, we do not need to use alpha's destination register.
This fixes an invalid "Too many hardware temporaries used" error in
the case where a transcendent operation writes to a temporary register
greater than max_temp_regs.

NOTE: This is a candidate for the 7.9 branch.