RISC-V: Support variable index in vec_extract.

RISC-V: Support variable index in vec_extract.

This patch adds a gen_lowpart in the vec_extract expander so it properly
works with a variable index and adds tests.

gcc/ChangeLog:

	* config/riscv/autovec.md: Add gen_lowpart.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/vls-vlmax/vec_extract-1.c: Add
	tests for variable index.
	* gcc.target/riscv/rvv/autovec/vls-vlmax/vec_extract-2.c: Ditto.
	* gcc.target/riscv/rvv/autovec/vls-vlmax/vec_extract-3.c: Ditto.
	* gcc.target/riscv/rvv/autovec/vls-vlmax/vec_extract-4.c: Ditto.
	* gcc.target/riscv/rvv/autovec/vls-vlmax/vec_extract-run.c:
	Ditto.
	* gcc.target/riscv/rvv/autovec/vls-vlmax/vec_extract-zvfh-run.c:
	Ditto.