glsl: do second pass of const propagation in loops

glsl: do second pass of const propagation in loops

When handling loops in constant propagation, implement the "FINISHME"
comment like copy propagation: perform a first pass to find values
that can't be propagated, then perform a second pass with the ACP
containing still valid values.

Certain values are killed because the loop may run more than one
iteration, so we can't copy propagate them as they would be invalid in
the later iterations.

Reviewed-by: Eric Anholt <eric@anholt.net>