RISC-V: Allow CONST_VECTOR for VLS modes

This patch enables COSNT_VECTOR for VLS modes.

void foo1 (int * __restrict a)
{
    for (int i = 0; i < 16; i++)
      a[i] = 8;
}

void foo2 (int * __restrict a)
{
    for (int i = 0; i < 16; i++)
      a[i] = i;
}

Compile option: -O3 --param=riscv-autovec-preference=scalable

Before this patch:

foo1:
        lui     a5,%hi(.LC0)
        addi    a5,a5,%lo(.LC0)
        vsetivli        zero,4,e32,m1,ta,ma
        addi    a4,a0,16
        vle32.v v1,0(a5)
        vse32.v v1,0(a0)
        vse32.v v1,0(a4)
        addi    a4,a0,32
        vse32.v v1,0(a4)
        addi    a0,a0,48
        vse32.v v1,0(a0)
        ret
foo2:
        lui     a5,%hi(.LC1)
        addi    a5,a5,%lo(.LC1)
        vsetivli        zero,4,e32,m1,ta,ma
        vle32.v v1,0(a5)
        lui     a5,%hi(.LC2)
        addi    a5,a5,%lo(.LC2)
        vse32.v v1,0(a0)
        vle32.v v1,0(a5)
        lui     a5,%hi(.LC3)
        addi    a4,a0,16
        addi    a5,a5,%lo(.LC3)
        vse32.v v1,0(a4)
        vle32.v v1,0(a5)
        addi    a4,a0,32
        lui     a5,%hi(.LC4)
        vse32.v v1,0(a4)
        addi    a0,a0,48
        addi    a5,a5,%lo(.LC4)
        vle32.v v1,0(a5)
        vse32.v v1,0(a0)
        ret

After this patch:

foo1:
	vsetivli	zero,16,e32,mf2,ta,ma
	vmv.v.i	v1,8
	vse32.v	v1,0(a0)
	ret
	.size	foo1, .-foo1
	.align	1
	.globl	foo2
	.type	foo2, @function
foo2:
	vsetivli	zero,16,e32,mf2,ta,ma
	vid.v	v1
	vse32.v	v1,0(a0)
	ret

gcc/ChangeLog:

	* config/riscv/autovec.md: Add VLS CONST_VECTOR.
	* config/riscv/riscv.cc (riscv_const_insns): Ditto.
	* config/riscv/vector.md: Ditto.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/vls/def.h: Add VLS CONST_VECTOR tests.
	* gcc.target/riscv/rvv/autovec/vls/const-1.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/const-2.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/const-3.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/const-4.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/const-5.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/series-1.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/series-2.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/series-3.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/series-4.c: New test.