RISC-V: Enable basic RVV auto-vectorization support.

RISC-V: Enable basic RVV auto-vectorization support.

gcc/ChangeLog:

	* config/riscv/riscv-protos.h (preferred_simd_mode): New function.
	* config/riscv/riscv-v.cc (autovec_use_vlmax_p): Ditto.
	(preferred_simd_mode): Ditto.
	* config/riscv/riscv.cc (riscv_get_arg_info): Handle RVV type in function arg.
	(riscv_convert_vector_bits): Adjust for RVV auto-vectorization.
	(riscv_preferred_simd_mode): New function.
	(TARGET_VECTORIZE_PREFERRED_SIMD_MODE): New target hook support.
	* config/riscv/vector.md: Add autovec.md.
	* config/riscv/autovec.md: New file.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/rvv.exp: Add testcases for RVV auto-vectorization.
	* gcc.target/riscv/rvv/autovec/fixed-vlmax-1.c: New test.
	* gcc.target/riscv/rvv/autovec/partial/single_rgroup-1.c: New test.
	* gcc.target/riscv/rvv/autovec/partial/single_rgroup-1.h: New test.
	* gcc.target/riscv/rvv/autovec/partial/single_rgroup_run-1.c: New test.
	* gcc.target/riscv/rvv/autovec/scalable-1.c: New test.
	* gcc.target/riscv/rvv/autovec/template-1.h: New test.
	* gcc.target/riscv/rvv/autovec/v-1.c: New test.
	* gcc.target/riscv/rvv/autovec/v-2.c: New test.
	* gcc.target/riscv/rvv/autovec/zve32f-1.c: New test.
	* gcc.target/riscv/rvv/autovec/zve32f-2.c: New test.
	* gcc.target/riscv/rvv/autovec/zve32f-3.c: New test.
	* gcc.target/riscv/rvv/autovec/zve32f_zvl128b-1.c: New test.
	* gcc.target/riscv/rvv/autovec/zve32f_zvl128b-2.c: New test.
	* gcc.target/riscv/rvv/autovec/zve32x-1.c: New test.
	* gcc.target/riscv/rvv/autovec/zve32x-2.c: New test.
	* gcc.target/riscv/rvv/autovec/zve32x-3.c: New test.
	* gcc.target/riscv/rvv/autovec/zve32x_zvl128b-1.c: New test.
	* gcc.target/riscv/rvv/autovec/zve32x_zvl128b-2.c: New test.
	* gcc.target/riscv/rvv/autovec/zve64d-1.c: New test.
	* gcc.target/riscv/rvv/autovec/zve64d-2.c: New test.
	* gcc.target/riscv/rvv/autovec/zve64d-3.c: New test.
	* gcc.target/riscv/rvv/autovec/zve64d_zvl128b-1.c: New test.
	* gcc.target/riscv/rvv/autovec/zve64d_zvl128b-2.c: New test.
	* gcc.target/riscv/rvv/autovec/zve64f-1.c: New test.
	* gcc.target/riscv/rvv/autovec/zve64f-2.c: New test.
	* gcc.target/riscv/rvv/autovec/zve64f-3.c: New test.
	* gcc.target/riscv/rvv/autovec/zve64f_zvl128b-1.c: New test.
	* gcc.target/riscv/rvv/autovec/zve64f_zvl128b-2.c: New test.
	* gcc.target/riscv/rvv/autovec/zve64x-1.c: New test.
	* gcc.target/riscv/rvv/autovec/zve64x-2.c: New test.
	* gcc.target/riscv/rvv/autovec/zve64x-3.c: New test.
	* gcc.target/riscv/rvv/autovec/zve64x_zvl128b-1.c: New test.
	* gcc.target/riscv/rvv/autovec/zve64x_zvl128b-2.c: New test.