st/mesa: keep track of saturated writes when eliminating dead code

st/mesa: keep track of saturated writes when eliminating dead code

It doesn't matter whether a write is saturated or not, in another
implementation it might even have been a separate opcode. This code was
most likely copied from the copy-propagation pass (where one does have
to distinguish saturation).

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>