RISC-V: Allow dest operand and accumulator operand overlap of widen reduction instruc...

RISC-V: Allow dest operand and accumulator operand overlap of widen reduction instruction[PR112327]

Consider this following intrinsic code:

void rvv_dot_prod(int16_t *pSrcA, int16_t *pSrcB, uint32_t n, int64_t *result)
{
    size_t vl;
    vint16m4_t vSrcA, vSrcB;
    vint64m1_t vSum = __riscv_vmv_s_x_i64m1(0, 1);
    while (n > 0) {
        vl = __riscv_vsetvl_e16m4(n);
        vSrcA = __riscv_vle16_v_i16m4(pSrcA, vl);
        vSrcB = __riscv_vle16_v_i16m4(pSrcB, vl);
        vSum = __riscv_vwredsum_vs_i32m8_i64m1(__riscv_vwmul_vv_i32m8(vSrcA, vSrcB, vl), vSum, vl);
        pSrcA += vl;
        pSrcB += vl;
        n -= vl;
    }
    *result = __riscv_vmv_x_s_i64m1_i64(vSum);
}

https://godbolt.org/z/vWd35W7G6

Before this patch:

...
Loop:
...
vmv1r.v v2,v1
...
vwredsum.vs     v1,v8,v2
...

After this patch:

...
Loop:
...
vwredsum.vs	v1,v8,v1
...

	PR target/112327

gcc/ChangeLog:

	* config/riscv/vector.md: Add '0'.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/base/pr112327-1.c: New test.
	* gcc.target/riscv/rvv/base/pr112327-2.c: New test.