aarch64: Fix loose ldpstp check [PR111411]

aarch64: Fix loose ldpstp check [PR111411]

aarch64_operands_ok_for_ldpstp contained the code:

  /* One of the memory accesses must be a mempair operand.
     If it is not the first one, they need to be swapped by the
     peephole.  */
  if (!aarch64_mem_pair_operand (mem_1, GET_MODE (mem_1))
       && !aarch64_mem_pair_operand (mem_2, GET_MODE (mem_2)))
    return false;

But the requirement isn't just that one of the accesses must be a
valid mempair operand.  It's that the lower access must be, since
that's the access that will be used for the instruction operand.

gcc/
	PR target/111411
	* config/aarch64/aarch64.cc (aarch64_operands_ok_for_ldpstp): Require
	the lower memory access to a mem-pair operand.

gcc/testsuite/
	PR target/111411
	* gcc.dg/rtl/aarch64/pr111411.c: New test.

(cherry picked from commit 2d38f45bcca62ca0c7afef4b579f82c5c2a01610)