RISC-V: Fix RVV register order

RISC-V: Fix RVV register order

This patch fixes the issue of incorrect reigster order of RVV.
The new register order is coming from kito original RVV GCC implementation.

Consider this case:
void f (void *base,void *base2,void *out,size_t vl, int n)
{
    vuint64m8_t bindex = __riscv_vle64_v_u64m8 (base + 100, vl);
    for (int i = 0; i < n; i++){
      vbool8_t m = __riscv_vlm_v_b8 (base + i, vl);
      vuint64m8_t v = __riscv_vluxei64_v_u64m8_m(m,base,bindex,vl);
      vuint64m8_t v2 = __riscv_vle64_v_u64m8_tu (v, base2 + i, vl);
      vint8m1_t v3 = __riscv_vluxei64_v_i8m1_m(m,base,v,vl);
      vint8m1_t v4 = __riscv_vluxei64_v_i8m1_m(m,base,v2,vl);
      __riscv_vse8_v_i8m1 (out + 100*i,v3,vl);
      __riscv_vse8_v_i8m1 (out + 222*i,v4,vl);
    }
}

Before this patch:
f:
	csrr    t0,vlenb
	slli    t1,t0,3
	sub     sp,sp,t1
	addi    a5,a0,100
	vsetvli zero,a3,e64,m8,ta,ma
	vle64.v v24,0(a5)
	vs8r.v  v24,0(sp)
	ble     a4,zero,.L1
	mv      a6,a0
	add     a4,a4,a0
	mv      a5,a2
.L3:
	vsetvli zero,zero,e64,m8,ta,ma
	vl8re64.v       v24,0(sp)
	vlm.v   v0,0(a6)
	vluxei64.v      v24,(a0),v24,v0.t
	addi    a6,a6,1
	vsetvli zero,zero,e8,m1,tu,ma
	vmv8r.v v16,v24
	vluxei64.v      v8,(a0),v24,v0.t
	vle64.v v16,0(a1)
	vluxei64.v      v24,(a0),v16,v0.t
	vse8.v  v8,0(a2)
	vse8.v  v24,0(a5)
	addi    a1,a1,1
	addi    a2,a2,100
	addi    a5,a5,222
	bne     a4,a6,.L3
.L1:
	csrr    t0,vlenb
	slli    t1,t0,3
	add     sp,sp,t1
	jr      ra

After this patch:
f:
	addi    a5,a0,100
	vsetvli zero,a3,e64,m8,ta,ma
	vle64.v v24,0(a5)
	ble     a4,zero,.L1
	mv      a6,a0
	add     a4,a4,a0
	mv      a5,a2
.L3:
	vsetvli zero,zero,e64,m8,ta,ma
	vlm.v   v0,0(a6)
	addi    a6,a6,1
	vluxei64.v      v8,(a0),v24,v0.t
	vsetvli zero,zero,e8,m1,tu,ma
	vmv8r.v v16,v8
	vluxei64.v      v2,(a0),v8,v0.t
	vle64.v v16,0(a1)
	vluxei64.v      v1,(a0),v16,v0.t
	vse8.v  v2,0(a2)
	vse8.v  v1,0(a5)
	addi    a1,a1,1
	addi    a2,a2,100
	addi    a5,a5,222
	bne     a4,a6,.L3
.L1:
	ret

The redundant register spillings is eliminated.
However, there is one more issue need to be addressed which is the redundant
move instruction "vmv8r.v". This is another story, and it will be fixed by another
patch (Fine tune RVV machine description RA constraint).

gcc/ChangeLog:

	* config/riscv/riscv.h (enum reg_class): Fix RVV register order.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/base/spill-4.c: Adapt testcase.
	* gcc.target/riscv/rvv/base/spill-6.c: Adapt testcase.
	* gcc.target/riscv/rvv/base/reg_order-1.c: New test.

Signed-off-by: Ju-Zhe Zhong <juzhe.zhong@rivai.ai>
Co-authored-by: kito-cheng <kito.cheng@sifive.com>