Draft: r300: after writing to temp variable, use write mask as a swizzle for reading

After reading #352 (closed) there was strong suggestion that lrp is implemented in wrong way. Even though I'd spend some time reading some code I wasn't able to track anything wrong, so I started reading debug output from tests which are using lrp.

In spec@glsl-1.20@execution@fs-mix-1.0

 34:   LRP temp[5].x, temp[7].yyyy, temp[4].xxxx, const[4].xxxx;

becomes:

 37: ADD temp[5].x, temp[4].xxxx, -const[4].xxxx;
 38: MAD temp[5].x, temp[7].yyyy, temp[5], const[4].xxxx;

As you can see it's possible to have bad swizzle on temp. This MR should cover this issue (though I'm not 100% sure what is happening with trigonometry stuff aka code which looks ok).

Edit: Todo: Rename commits.