Power10: Add PCREL_OPT store support.

Power10: Add PCREL_OPT store support.

gcc/
2020-09-15  Michael Meissner  <meissner@linux.ibm.com>

	* config/rs6000/pcrel-opt.c (pcrel_opt_store): New function.
	(pcrel_opt_address): Add PCREL_OPT support for stores.
	(pcrel_opt_pass): Print PCREL_OPT store statistics.
	* config/rs6000/pcrel-opt.md (UNSPEC_PCREL_OPT_ST_ADDR): New
	unspec.
	(UNSPEC_PCREL_OPT_ST_RELOC): New unspec.
	(pcrel_opt_st_addr<mode>): New insns for PCREL_OPT store support.
	(pcrel_opt_st<mode>, QHSI iterator): New insns for PCREL_OPT store
	support.
	(pcrel_opt_stdi): New insn for PCREL_OPT store support.
	(pcrel_opt_stsf): New insn for PCREL_OPT store support.
	(pcrel_opt_stdf): New insns for PCREL_OPT store support.
	(pcrel_opt_st<mode>, PO_VECT iterator):  New insns for PCREL_OPT
	store support.
	* config/rs6000/rs6000.c (rs6000_delegitimize_address): Add
	support for PCREL_OPT store.