RISC-V: Add testcase for vrsub.vi auto-vectorization

Apparently, we are missing vrsub.vi tests.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/binop/vsub-run.c: Add vsub.vi.
	* gcc.target/riscv/rvv/autovec/binop/vsub-rv32gcv.c: Ditto.
	* gcc.target/riscv/rvv/autovec/binop/vsub-rv64gcv.c: Ditto.
	* gcc.target/riscv/rvv/autovec/binop/vsub-template.h: Ditto.

Signed-off-by: Juzhe-Zhong <juzhe.zhong@rivai.ai>