RISC-V: Add vandn combine helper.

RISC-V: Add vandn combine helper.

This patch adds a combine pattern for vandn as well as tests for it.

gcc/ChangeLog:

	* config/riscv/autovec-opt.md (*vandn_<mode>): New pattern.
	* config/riscv/vector.md: Add vandn to mode_idx.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/binop/vandn-1.c: New test.
	* gcc.target/riscv/rvv/autovec/binop/vandn-run.c: New test.
	* gcc.target/riscv/rvv/autovec/binop/vandn-template.h: New test.