RISC-V: Add test for vec_duplicate + vrsub.vv combine case 0 with GR2VR cost 15

RISC-V: Add test for vec_duplicate + vrsub.vv combine case 0 with GR2VR cost 15

Add asm dump check test for vec_duplicate + vrsub.vv combine to vrsub.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-3-i16.c: Add asm check
	for vrsub with GR2VR cost is 15.
	* gcc.target/riscv/rvv/autovec/vx_vf/vx-3-i32.c: Ditto.
	* gcc.target/riscv/rvv/autovec/vx_vf/vx-3-i64.c: Ditto.
	* gcc.target/riscv/rvv/autovec/vx_vf/vx-3-i8.c: Ditto.
	* gcc.target/riscv/rvv/autovec/vx_vf/vx-3-u16.c: Ditto.
	* gcc.target/riscv/rvv/autovec/vx_vf/vx-3-u32.c: Ditto.
	* gcc.target/riscv/rvv/autovec/vx_vf/vx-3-u64.c: Ditto.
	* gcc.target/riscv/rvv/autovec/vx_vf/vx-3-u8.c: Ditto.

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