RISC-V: Add xfail test case for widening register overlap of vf4/vf8

RISC-V: Add xfail test case for widening register overlap of vf4/vf8

We reverted below patch for register group overlap, add the related
insn test and mark it as xfail.  And we will remove the xfail
after we support the register overlap in GCC-15.

303195e2a6b RISC-V: Support widening register overlap for vf4/vf8

The below test suites are passed.
* The rv64gcv fully regression test.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/base/pr112431-16.c: New test.
	* gcc.target/riscv/rvv/base/pr112431-17.c: New test.
	* gcc.target/riscv/rvv/base/pr112431-18.c: New test.

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