REG_N_REFS and REG_N_SETS are initialized by a call to
regstat_init_n_sets_and_refs from the current values of
DF_REG_DEF_COUNT and DF_REG_USE_COUNT. REG_N_REFS and REG_N_SETS
should only be used if a pass need to change these values in some
magical way or the pass needs to have accurate values for these
and is not using incremental df scanning.
At the end of a pass that uses REG_N_REFS and REG_N_SETS, a call
should be made to regstat_free_n_sets_and_refs.
Local alloc seems to play pretty loose with these values.
REG_N_REFS is set to 0 if the register is used in an asm.
Furthermore, local_alloc calls regclass to hack both REG_N_REFS and
REG_N_SETS for three address insns. Other passes seem to have
other special values.
Structure to hold values for REG_N_SETS (i) and REG_N_REFS (i).