RISC-V: Add test for vec_duplicate + vsub.vv combine case 1 with GR2VR cost 2

RISC-V: Add test for vec_duplicate + vsub.vv combine case 1 with GR2VR cost 2

Add asm dump check test for vec_duplicate + vsub.vv combine to vsub.vx.

The below test suites are passed for this patch.
* The rv64gcv fully regression test.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/vx_vf/vx-6-i16.c: Add test cases
	for vsub vx combine case 1 with GR2VR cost 2.
	* gcc.target/riscv/rvv/autovec/vx_vf/vx-6-i32.c: Ditto.
	* gcc.target/riscv/rvv/autovec/vx_vf/vx-6-i64.c: Ditto.
	* gcc.target/riscv/rvv/autovec/vx_vf/vx-6-i8.c: Ditto.
	* gcc.target/riscv/rvv/autovec/vx_vf/vx-6-u16.c: Ditto.
	* gcc.target/riscv/rvv/autovec/vx_vf/vx-6-u32.c: Ditto.
	* gcc.target/riscv/rvv/autovec/vx_vf/vx-6-u64.c: Ditto.
	* gcc.target/riscv/rvv/autovec/vx_vf/vx-6-u8.c: Ditto.

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