nir: add pass to move load_const

Run this pass late (after opt loop) to move load_const instructions back
into the basic blocks which use the result, in cases where a load_const
is only consumed in a single block.

This helps reduce register usage in cases where the backend driver
cannot lower the load_const to a uniform.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/compiler/nir/nir.h

@@ -2632,6 +2632,7 @@ bool nir_remove_dead_variables(nir_shader *shader, nir_variable_mode modes);
 bool nir_lower_constant_initializers(nir_shader *shader,
                                      nir_variable_mode modes);
 
+bool nir_move_load_const(nir_shader *shader);
 bool nir_move_vec_src_uses_to_dest(nir_shader *shader);
 bool nir_lower_vec_to_movs(nir_shader *shader);
 void nir_lower_alpha_test(nir_shader *shader, enum compare_func func,