RISC-V: Add test cases for vwmulu.vx reg overlap

Add test cases for register group overlap, please
note it is not overlap as much as possible.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/group_overlap/vwmulu_vx-u16-m1.c: New test.
	* gcc.target/riscv/rvv/autovec/group_overlap/vwmulu_vx-u16-m2.c: New test.
	* gcc.target/riscv/rvv/autovec/group_overlap/vwmulu_vx-u16-m4.c: New test.
	* gcc.target/riscv/rvv/autovec/group_overlap/vwmulu_vx-u16-mf2.c: New test.
	* gcc.target/riscv/rvv/autovec/group_overlap/vwmulu_vx-u16-mf4.c: New test.
	* gcc.target/riscv/rvv/autovec/group_overlap/vwmulu_vx-u32-m1.c: New test.
	* gcc.target/riscv/rvv/autovec/group_overlap/vwmulu_vx-u32-m2.c: New test.
	* gcc.target/riscv/rvv/autovec/group_overlap/vwmulu_vx-u32-m4.c: New test.
	* gcc.target/riscv/rvv/autovec/group_overlap/vwmulu_vx-u32-mf2.c: New test.
	* gcc.target/riscv/rvv/autovec/group_overlap/vwmulu_vx-u8-m1.c: New test.
	* gcc.target/riscv/rvv/autovec/group_overlap/vwmulu_vx-u8-m2.c: New test.
	* gcc.target/riscv/rvv/autovec/group_overlap/vwmulu_vx-u8-m4.c: New test.
	* gcc.target/riscv/rvv/autovec/group_overlap/vwmulu_vx-u8-mf2.c: New test.
	* gcc.target/riscv/rvv/autovec/group_overlap/vwmulu_vx-u8-mf4.c: New test.
	* gcc.target/riscv/rvv/autovec/group_overlap/vwmulu_vx-u8-mf8.c: New test.

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