nir/gcm: Consider dead code elimination done by GCM as progress

This will also fix NIR_DEBUG=extended_validation complaining about
invalid loop analysis.  GCM will invalidate loop analysis if progress
was made, and depending on the removed instruction it will affect the
instr_cost.

Cc: mesa-stable
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/38932>

src/compiler/nir/nir_opt_gcm.c

@@ -759,6 +759,7 @@ gcm_place_instr(nir_instr *instr, struct gcm_state *state)
    if (instr->block == NULL) {
       nir_foreach_def(instr, gcm_replace_def_with_undef, state);
       nir_instr_remove(instr);
+      state->progress = true;
       return;
    }