RISC-V: Add xfail test case for highpart overlap of vext.vf

RISC-V: Add xfail test case for highpart overlap of vext.vf

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.

62685890d88 RISC-V: Support highpart overlap for vext.vf

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/unop_v_constraint-2.c: Adjust asm
	check cond.
	* gcc.target/riscv/rvv/base/pr112431-4.c: New test.
	* gcc.target/riscv/rvv/base/pr112431-5.c: New test.
	* gcc.target/riscv/rvv/base/pr112431-6.c: New test.

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