glsl: Add a new opt_copy_propagation variant that does it channel-wise.

glsl: Add a new opt_copy_propagation variant that does it channel-wise.

This patch cleans up many of the extra copies in GLSL IR introduced by
i965's scalarizing passes.  It doesn't result in a statistically
significant performance difference on nexuiz high settings (n=3) or my
demo (n=10), due to brw_fs.cpp's register coalescing covering most of
those extra moves anyway.  However, it does make the debug of wine's
GLSL shaders much more tractable, and reduces instruction count of
glsl-fs-convolution-2 from 376 to 288.