rs6000: Teach legitimate_address_p about LEN_{LOAD,STORE} [PR110248]

rs6000: Teach legitimate_address_p about LEN_{LOAD,STORE} [PR110248]

This patch is to teach rs6000_legitimate_address_p to
handle the queried rtx constructed for LEN_{LOAD,STORE},
since lxvl and stxvl doesn't support x-form or ds-form,
so consider it as not legitimate when outer code is PLUS.

	PR tree-optimization/110248

gcc/ChangeLog:

	* config/rs6000/rs6000.cc (rs6000_legitimate_address_p): Check if
	the given code is for ifn LEN_{LOAD,STORE}, if yes then make it not
	legitimate when outer code is PLUS.