RISC-V: Add test for vec_duplicate + vaaddu.vv combine for DImode

RISC-V: Add test for vec_duplicate + vaaddu.vv combine for DImode

Add asm dump check and run test for vec_duplicate + vaaddu.vv
combine to vaaddu.vx, with the GR2VR cost is 0, 1, 2 and 15 for
the case 0 and case 1.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/vx_vf/vx-1-u64.c: Add asm check.
	* gcc.target/riscv/rvv/autovec/vx_vf/vx-2-u64.c: Ditto.
	* gcc.target/riscv/rvv/autovec/vx_vf/vx-3-u64.c: Ditto.
	* gcc.target/riscv/rvv/autovec/vx_vf/vx-4-u64.c: Ditto.
	* gcc.target/riscv/rvv/autovec/vx_vf/vx-5-u64.c: Ditto.
	* gcc.target/riscv/rvv/autovec/vx_vf/vx-6-u64.c: Ditto.
	* gcc.target/riscv/rvv/autovec/vx_vf/vx_vaadd-run-1-u64.c: New test.

Signed-off-by: Pan Li <pan2.li@intel.com>