RISC-V: Support TARGET_VECTORIZE_PREFERRED_VECTOR_ALIGNMENT to optimize codegen of...

RISC-V: Support TARGET_VECTORIZE_PREFERRED_VECTOR_ALIGNMENT to optimize codegen of both VLA && VLS auto-vectorization

This patch optimizes both RVV VLA && VLS vectorization.

Consider this following case:
void __attribute__((noinline, noclone))
f (int * __restrict dst, int * __restrict op1, int * __restrict op2, int
count)
{
  for (int i = 0; i < count; ++i)
    dst[i] = op1[i] + op2[i];
}

VLA:
Before this patch:
        ble	a3,zero,.L1
        srli	a4,a1,2
        negw	a4,a4
        andi	a5,a4,3
        sext.w	a3,a3
        beq	a5,zero,.L3
        lw	a7,0(a1)
        lw	a6,0(a2)
        andi	a4,a4,2
        addw	a6,a6,a7
        sw	a6,0(a0)
        beq	a4,zero,.L3
        lw	a7,4(a1)
        lw	a4,4(a2)
        li	a6,3
        addw	a4,a4,a7
        sw	a4,4(a0)
        bne	a5,a6,.L3
        lw	a6,8(a2)
        lw	a4,8(a1)
        addw	a4,a4,a6
        sw	a4,8(a0)
.L3:
        subw	a3,a3,a5
        slli	a4,a3,32
        csrr	a6,vlenb
        srli	a4,a4,32
        srli	a6,a6,2
        slli	a3,a5,2
        mv	a5,a4
        bgtu	a4,a6,.L17
.L5:
        csrr	a6,vlenb
        add	a1,a1,a3
        add	a2,a2,a3
        add	a0,a0,a3
        srli	a7,a6,2
        li	a3,0
.L8:
        vsetvli	zero,a5,e32,m1,ta,ma
        vle32.v	v1,0(a1)
        vle32.v	v2,0(a2)
        vsetvli	t1,zero,e32,m1,ta,ma
        add	a3,a3,a7
        vadd.vv	v1,v1,v2
        vsetvli	zero,a5,e32,m1,ta,ma
        vse32.v	v1,0(a0)
        mv	a5,a4
        bleu	a4,a3,.L6
        mv	a5,a3
.L6:
        sub	a5,a4,a5
        bleu	a5,a7,.L7
        mv	a5,a7
.L7:
        add	a1,a1,a6
        add	a2,a2,a6
        add	a0,a0,a6
        bne	a5,zero,.L8
.L1:
        ret
.L17:
        mv	a5,a6
        j	.L5

After this patch:
f:
        ble     a3,zero,.L1
        csrr    a4,vlenb
        srli    a4,a4,2
        mv      a5,a3
        bgtu    a3,a4,.L9
.L3:
        csrr    a6,vlenb
        li      a4,0
        srli    a7,a6,2
.L6:
        vsetvli zero,a5,e32,m1,ta,ma
        vle32.v v2,0(a1)
        vle32.v v1,0(a2)
        vsetvli t1,zero,e32,m1,ta,ma
        add     a4,a4,a7
        vadd.vv v1,v1,v2
        vsetvli zero,a5,e32,m1,ta,ma
        vse32.v v1,0(a0)
        mv      a5,a3
        bleu    a3,a4,.L4
        mv      a5,a4
.L4:
        sub     a5,a3,a5
        bleu    a5,a7,.L5
        mv      a5,a7
.L5:
        add     a0,a0,a6
        add     a2,a2,a6
        add     a1,a1,a6
        bne     a5,zero,.L6
.L1:
        ret
.L9:
        mv      a5,a4
        j       .L3

VLS:
Before this patch:
f3:
        ble	a3,zero,.L1
        srli	a5,a1,2
        negw	a5,a5
        andi	a4,a5,3
        sext.w	a3,a3
        beq	a4,zero,.L3
        lw	a7,0(a1)
        lw	a6,0(a2)
        andi	a5,a5,2
        addw	a6,a6,a7
        sw	a6,0(a0)
        beq	a5,zero,.L3
        lw	a7,4(a1)
        lw	a5,4(a2)
        li	a6,3
        addw	a5,a5,a7
        sw	a5,4(a0)
        bne	a4,a6,.L3
        lw	a6,8(a2)
        lw	a5,8(a1)
        addw	a5,a5,a6
        sw	a5,8(a0)
.L3:
        subw	a3,a3,a4
        slli	a6,a4,2
        slli	a5,a3,32
        srli	a5,a5,32
        add	a1,a1,a6
        add	a2,a2,a6
        add	a0,a0,a6
        li	a3,4
.L6:
        mv	a4,a5
        bleu	a5,a3,.L5
        li	a4,4
.L5:
        vsetvli	zero,a4,e32,m1,ta,ma
        vle32.v	v1,0(a1)
        vle32.v	v2,0(a2)
        vsetivli	zero,4,e32,m1,ta,ma
        sub	a5,a5,a4
        vadd.vv	v1,v1,v2
        vsetvli	zero,a4,e32,m1,ta,ma
        vse32.v	v1,0(a0)
        addi	a1,a1,16
        addi	a2,a2,16
        addi	a0,a0,16
        bne	a5,zero,.L6
.L1:
        ret

After this patch:
f3:
        ble	a3,zero,.L1
        li	a4,4
.L4:
        mv	a5,a3
        bleu	a3,a4,.L3
        li	a5,4
.L3:
        vsetvli	zero,a5,e32,m1,ta,ma
        vle32.v	v2,0(a1)
        vle32.v	v1,0(a2)
        vsetivli	zero,4,e32,m1,ta,ma
        sub	a3,a3,a5
        vadd.vv	v1,v1,v2
        vsetvli	zero,a5,e32,m1,ta,ma
        vse32.v	v1,0(a0)
        addi	a2,a2,16
        addi	a0,a0,16
        addi	a1,a1,16
        bne	a3,zero,.L4
.L1:
        ret

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

	* config/riscv/riscv.cc
	(riscv_vectorize_preferred_vector_alignment): New function.
	(TARGET_VECTORIZE_PREFERRED_VECTOR_ALIGNMENT): New target hook.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/binop/shift-rv32gcv.c: Adapt testcase.
	* gcc.target/riscv/rvv/autovec/align-1.c: New test.
	* gcc.target/riscv/rvv/autovec/align-2.c: New test.