freedreno/ir3/cf: handle multiple cov's properly

freedreno/ir3/cf: handle multiple cov's properly

There can be multiple (for ex.) f32f16's from a single source, in
particular appearing in different blocks.  We need to update all uses
of the src which had conversion folded in, not all the uses of the
individual cov.  Also, to avoid invalidating the ssa use info that was
gathered at the beginning of the pass, don't actually eliminate the
cov, but instead change it to a simple mov that the cp pass can gobble
up.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5048>