GIMPLE_FOLD: Apply LEN_MASK_{LOAD,STORE} into GIMPLE_FOLD

GIMPLE_FOLD: Apply LEN_MASK_{LOAD,STORE} into GIMPLE_FOLD

Hi, since we are going to have LEN_MASK_{LOAD,STORE} into loopVectorizer.

Currenly,
1. we can fold MASK_{LOAD,STORE} into MEM when mask is all ones.
2. we can fold LEN_{LOAD,STORE} into MEM when (len - bias) is VF.

Now, I think it makes sense that we can support

fold LEN_MASK_{LOAD,STORE} into MEM when both mask = all ones and (len - bias) is VF.

gcc/ChangeLog:

	* gimple-fold.cc (arith_overflowed_p): Apply LEN_MASK_{LOAD,STORE}.
	(gimple_fold_partial_load_store_mem_ref): Ditto.
	(gimple_fold_partial_store): Ditto.
	(gimple_fold_call): Ditto.