freedreno/ir3: better cleanup when removing unused instructions

freedreno/ir3: better cleanup when removing unused instructions

Do a better job of pruning when removing unused instructions, including
cleaning up dangling false-deps.

This introduces a new ssa src pointer iterator, which makes it easy to
clear links without having to think about whether it is a normal ssa
src or a false-dep.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4440>