nir/opt_if: Remove unneeded phis if we make progress

Now that SSA values can be derefs and they have special rules, we have
to be a bit more careful about our LCSSA phis.  In particular, we need
to clean up in case LCSSA ended up creating a phi node for a deref.
This fixes validation issues with some Vulkan CTS tests with the new
deref instructions.

Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>

src/compiler/nir/nir.h

@@ -2903,6 +2903,7 @@ bool nir_opt_move_load_ubo(nir_shader *shader);
 
 bool nir_opt_peephole_select(nir_shader *shader, unsigned limit);
 
+bool nir_opt_remove_phis_impl(nir_function_impl *impl);
 bool nir_opt_remove_phis(nir_shader *shader);
 
 bool nir_opt_shrink_load(nir_shader *shader);