RISC-V: Add xfail test case for highest-number regno ternary overlap

RISC-V: Add xfail test case for highest-number regno ternary overlap

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.

27fde325d64 RISC-V: Support highest-number regno overlap for widen ternary

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

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/base/pr112431-37.c: New test.
	* gcc.target/riscv/rvv/base/pr112431-38.c: New test.

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