Correctly identify stfs if prefixed

Correctly identify stfs if prefixed

Because reg_to_non_prefixed() only looks at the register being used, it
doesn't get the right answer for stfs, which leads to us not seeing
that it has a PCREL symbol ref.  This patch works around this by
introducing a helper function that inspects the insn to see if it is in
fact a stfs. Then if we use NON_PREFIXED_DEFAULT, address_to_insn_form()
can see that it has the PCREL symbol ref.

gcc/ChangeLog:

	PR target/95347
	* config/rs6000/rs6000.c (prefixed_store_p): Add special case
	for stfs.
	(is_stfs_insn): New helper function.