nir/gcm: Delete dead instructions

nir/gcm: Delete dead instructions

Classically, global code motion is also a dead code pass.  However, in
the initial implementation, the decision was made to place every
instruction and let conventional DCE clean up the dead ones.  Because
any uses of a dead instruction are unreachable, we have no late block
and the dead instructions are always scheduled early.  The problem is
that, because we place the dead instruction early, it  pushes the
placement of any dependencies of the dead instruction earlier than they
may need to be placed.  In order prevent dead instructions from
affecting the placement of live ones, we need to delete them.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4636>