hard-reg-set: use ctz for iteration

hard-reg-set: use ctz for iteration

Simplify the HARD_REG_SET iteration, using ctz and avoiding some
unnecessary operations.

for  gcc/ChangeLog

	* hard-reg-set.h (hard_reg_set_iter_init): Drop unnecessary
	increment of min.
	(hard_reg_set_iter_set): Use ctz_hwi, and compute
	word-advanced regno from word_no.
	(hard_reg_set_iter_next): Only clear the cached LSB.