glsl: Initialize source_chan in opt_copy_propagation_elements.

glsl: Initialize source_chan in opt_copy_propagation_elements.

Previously, if chans < 4, we passed uninitialized stack garbage to the
ir_swizzle constructor for the excess components.  Thankfully, it
ignores that data, as it's unnecessary, so no harm actually comes of it.

However, it's obviously better to initialize it.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>