RISC-V: Allow variable index for vec_set.

RISC-V: Allow variable index for vec_set.

This patch enables a variable index for vec_set and adjust the tests.

gcc/ChangeLog:

	* config/riscv/autovec.md: Allow register index operand.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/vls-vlmax/vec_set-1.c: Adjust
	test.
	* gcc.target/riscv/rvv/autovec/vls-vlmax/vec_set-2.c: Ditto.
	* gcc.target/riscv/rvv/autovec/vls-vlmax/vec_set-3.c: Ditto.
	* gcc.target/riscv/rvv/autovec/vls-vlmax/vec_set-4.c: Ditto.
	* gcc.target/riscv/rvv/autovec/vls-vlmax/vec_set-run.c: Ditto.
	* gcc.target/riscv/rvv/autovec/vls-vlmax/vec_set-zvfh-run.c:
	Ditto.