RISC-V: Add test cases for vfcvt.x.f.v reg overlap

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

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/group_overlap/vfwcvt_x-f16-m1.c: New test.
	* gcc.target/riscv/rvv/autovec/group_overlap/vfwcvt_x-f16-m2.c: New test.
	* gcc.target/riscv/rvv/autovec/group_overlap/vfwcvt_x-f16-m4.c: New test.
	* gcc.target/riscv/rvv/autovec/group_overlap/vfwcvt_x-f16-mf2.c: New test.
	* gcc.target/riscv/rvv/autovec/group_overlap/vfwcvt_x-f16-mf4.c: New test.
	* gcc.target/riscv/rvv/autovec/group_overlap/vfwcvt_x-f32-m1.c: New test.
	* gcc.target/riscv/rvv/autovec/group_overlap/vfwcvt_x-f32-m2.c: New test.
	* gcc.target/riscv/rvv/autovec/group_overlap/vfwcvt_x-f32-m4.c: New test.
	* gcc.target/riscv/rvv/autovec/group_overlap/vfwcvt_x-f32-mf2.c: New test.

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