glsl: optimize copy_propagation_elements pass

glsl: optimize copy_propagation_elements pass

Changes make copy_propagation_elements pass faster, reducing link
time spent in test case of bug 94477. Does not fix the actual issue
but brings down the total time. No regressions seen in CI.

v2 (idr): Formatting / whitespace fixes.  Embed the acp_ref in the
acp_entry.

v3 (idr): Delete unused copy constructor.  Use while(pop_head) instead
of foreach() { remove }.

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>