RISC-V: Enable basic VLS auto-vectorization

RISC-V: Enable basic VLS auto-vectorization

Consider this following case:
void
foo (int8_t *in, int8_t *out, int8_t x)
{
  for (int i = 0; i < 16; i++)
    in[i] = x;
}

Compile option: --param=riscv-autovec-preference=scalable -fno-builtin

Before this patch:

foo:
        li      a5,16
        csrr    a4,vlenb
        vsetvli a3,zero,e8,m1,ta,ma
        vmv.v.x v1,a2
        bleu    a5,a4,.L2
        mv      a5,a4
.L2:
        vsetvli zero,a5,e8,m1,ta,ma
        vse8.v  v1,0(a0)
        ret

After this patch:

foo:
	vsetivli	zero,16,e8,mf8,ta,ma
	vmv.v.x	v1,a2
	vse8.v	v1,0(a0)
	ret

gcc/ChangeLog:

	* config/riscv/autovec-vls.md (@vec_duplicate<mode>): New pattern.
	* config/riscv/riscv-v.cc (autovectorize_vector_modes): Add VLS autovec
	support.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/v-1.c: Adapt test.
	* gcc.target/riscv/rvv/autovec/zve32f_zvl128b-1.c: Ditto.
	* gcc.target/riscv/rvv/autovec/zve64d_zvl128b-1.c: Ditto.
	* gcc.target/riscv/rvv/autovec/zve64f_zvl128b-1.c: Ditto.
	* gcc.target/riscv/rvv/autovec/vls/dup-1.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/dup-2.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/dup-3.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/dup-4.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/dup-5.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/dup-6.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/dup-7.c: New test.