r300/compiler: Avoid generating MOV instructions for invalid IMM swizzles v2

r300/compiler: Avoid generating MOV instructions for invalid IMM swizzles v2

If an instruction reads from a constant register that contains
immediates using an invalid swizzle, we can avoid generating MOV
instructions to fix up the swizzle by loading the immediates into a
different constant register that can be read using a valid swizzle.

This only affects r300 and r400 cards.

For example:

CONST[1] = {    -3.5000     3.5000     2.5000     1.5000 }

MAD temp[4].xy, const[0].xy__, const[1].xz__, input[0].xy__;

========== Before this change would be lowered to: =========

CONST[1] = {    -3.5000     3.5000     2.5000     1.5000 }

MOV temp[0].x, const[1].x___;
MOV temp[0].y, const[1]._z__;
MAD temp[4].xy, const[0].xy__, temp[0].xy__, input[0].xy__;

========== After this change is lowered to:  ===============

CONST[1] = {    -3.5000     3.5000     2.5000     1.5000 }
CONST[2] = {     0.0000    -3.5000     2.5000     0.0000 }

MAD temp[4].xy, const[0].xy__, const[2].yz__, input[0].xy__;

============================================================

This change reduces one of the Lightsmark shaders from 133 to 91
instructions.

v2:
  - Fix crash caused by swizzles with only inline constants.