r300/compiler: Fix two mistakes in the presubtract optimization pass.

r300/compiler: Fix two mistakes in the presubtract optimization pass.

1. We can't turn an instruction into a presubtract operation if it
writes to one of the registers it reads from.
2. If we turn an instruction into a presubtract operation, we can't
remove that intruction unless all readers can use the presubtract
operation.

This fixes fdo bug 30337.
This is a candidate for the 7.9 branch.