mesa: add new program optimizer code

This is pretty simplistic for now, but helps with certain shaders.
diff --git a/src/mesa/SConscript b/src/mesa/SConscript
index b2f3d96..273ce11 100644
--- a/src/mesa/SConscript
+++ b/src/mesa/SConscript
@@ -193,6 +193,7 @@
 		'shader/prog_execute.c',
 		'shader/prog_instruction.c',
 		'shader/prog_noise.c',
+		'shader/prog_optimize.c',
 		'shader/prog_parameter.c',
 		'shader/prog_print.c',
 		'shader/prog_statevars.c',