nir/lower_reg: Remove dead reg_decl intrinsics

For any reg we can lower, we remove it whenever we remove the last read
or write.  For regs that aren't used at all, however, there are no reads
or writes so there's nothing to trigger the removal.  Instead, we need
to do it in setup_reg.

Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28300>

src/compiler/nir/nir_lower_reg_intrinsics_to_ssa.c

@@ -36,6 +36,11 @@ struct regs_to_ssa_state {
 static void
 setup_reg(nir_intrinsic_instr *decl, struct regs_to_ssa_state *state)
 {
+   if (nir_def_is_unused(&decl->def)) {
+      nir_instr_remove(&decl->instr);
+      return;
+   }
+
    assert(state->values[decl->def.index] == NULL);
    if (!should_lower_reg(decl))
       return;