RISC-V: Add xfail test case for highpart register overlap of vwcvt

RISC-V: Add xfail test case for highpart register overlap of vwcvt

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.

bdad036da32 RISC-V: Support highpart register overlap for vwcvt

The below test suites are passed for this patch
* The rv64gcv fully regression test with isl build.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/base/pr112431-1.c: New test.
	* gcc.target/riscv/rvv/base/pr112431-2.c: New test.
	* gcc.target/riscv/rvv/base/pr112431-3.c: New test.

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