RISC-V: Support highpart overlap for vext.vf

PR target/112431

gcc/ChangeLog:

	* config/riscv/vector.md: Support highpart overlap for vext.vf2

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/base/unop_v_constraint-2.c: Adapt test.
	* 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.