Copy propagation often eliminates all uses of an instruction. If we
detect that we've done so, we can eliminate the instruction ourselves
rather than leaving it hanging until the next DCE pass.
This saves some CPU time as other passes don't see dead code.
Reviewed-by: Caio Oliveira <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28666>