mesa: Add an optimization path to remove use of pointless MOVs.

mesa: Add an optimization path to remove use of pointless MOVs.

GLSL code such as:
	vec4 result = {0, 1, 0, 0};
	gl_FragColor = result;

emits code like:
  0: MOV TEMP[0], CONST[0];
  1: MOV OUTPUT[1], TEMP[0];

and this replaces it with:
  0: MOV TEMP[0], CONST[0];
  1: MOV OUTPUT[1], CONST[0];

Even when the dead code eliminator fails to clean up a now-useless MOV
instruction (since it doesn't do live/dead ranges), this should at reduce
dependencies.