RISC-V: Add testcases for vec_duplicate + vadd.vv combine when GR2VR cost 15

RISC-V: Add testcases for vec_duplicate + vadd.vv combine when GR2VR cost 15

Add asm dump check and for vec_duplicate + vadd.vv combine to vadd.vx.
The late-combine will not take action when GR2VR cost is 15.

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_vadd-3-i16.c: New test.
	* gcc.target/riscv/rvv/autovec/vx_vf/vx_vadd-3-i32.c: New test.
	* gcc.target/riscv/rvv/autovec/vx_vf/vx_vadd-3-i64.c: New test.
	* gcc.target/riscv/rvv/autovec/vx_vf/vx_vadd-3-i8.c: New test.
	* gcc.target/riscv/rvv/autovec/vx_vf/vx_vadd-3-u16.c: New test.
	* gcc.target/riscv/rvv/autovec/vx_vf/vx_vadd-3-u32.c: New test.
	* gcc.target/riscv/rvv/autovec/vx_vf/vx_vadd-3-u64.c: New test.
	* gcc.target/riscv/rvv/autovec/vx_vf/vx_vadd-3-u8.c: New test.

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