s390: Change SET rtx_cost handling.

s390: Change SET rtx_cost handling.

The IF_THEN_ELSE detection currently prevents us from properly costing
register-register moves which causes the lower-subreg pass to assume that
a VR-VR move is as expensive as two GPR-GPR moves.

This patch adds handling for SETs containing REGs as well as MEMs and is
inspired by the aarch64 implementation.

gcc/ChangeLog:

	* config/s390/s390.cc (s390_address_cost): Declare.
	(s390_hard_regno_nregs): Declare.
	(s390_rtx_costs): Add handling for REG and MEM in SET.

gcc/testsuite/ChangeLog:

	* gcc.target/s390/vector/vec-sum-across-no-lower-subreg-1.c: New test.