glsl: Make opt_copy_propagation_elements actually propagate into loops.

glsl: Make opt_copy_propagation_elements actually propagate into loops.

We've had a FINISHME here since Eric originally wrote the code in 2011.
This patch implements his suggested approach, which makes us actually
able to copy propagate into the loops, at the unfortunate cost of making
this pass even more expensive.

The shader-db statistics are basically a wash:

   No change in instruction counts.

   total cycles in shared programs: 78685980 -> 78680730 (-0.01%)
   cycles in affected programs: 2102646 -> 2097396 (-0.25%)
   helped: 48
   HURT: 83

I figured if we're going to do this for one copy propagation pass,
we may as well do it in both.

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