r300/compiler: Use consistent src swizzles for transcendent instructions

r300/compiler: Use consistent src swizzles for transcendent instructions

Source swizzles for transcendent instructions were being stored in the X
channel regardless of what channel the instruction was writing.
This was causing problems for some helper functions that were expecting
source swizzles to occupy channels corresponding to the instruction's
writemask.  This commit makes transcendent instructions follow the same
convention as normal instructions for representing source swizzles.

Previous behavior:
LG2 temp[0].y, input[0].x___;

Current behavior:
LG2 temp[0].y, input[0]._x__;