[ARC] Add peephole rules to combine store/loads into double store/loads

[ARC] Add peephole rules to combine store/loads into double store/loads

Simple peephole rules which combines multiple ld/st instructions into
64-bit load/store instructions. It only works for architectures which
are having double load/store option on.

gcc/
	Claudiu Zissulescu  <claziss@synopsys.com>

	* config/arc/arc-protos.h (gen_operands_ldd_std): Add.
	* config/arc/arc.c (operands_ok_ldd_std): New function.
	(mem_ok_for_ldd_std): Likewise.
	(gen_operands_ldd_std): Likewise.
	* config/arc/arc.md: Add peephole2 rules for std/ldd.

From-SVN: r266064