RISC-V: Make liveness estimation be aware of .vi variant

Consider this following case:

void
f (int *restrict a, int *restrict b, int *restrict c, int *restrict d, int n)
{
  for (int i = 0; i < n; i++)
    {
      int tmp = b[i] + 15;
      int tmp2 = tmp + b[i];
      c[i] = tmp2 + b[i];
      d[i] = tmp + tmp2 + b[i];
    }
}

Current dynamic LMUL cost model choose LMUL = 4 because we count the "15" as
consuming 1 vector register group which is not accurate.

We teach the dynamic LMUL cost model be aware of the potential vi variant instructions
transformation, so that we can choose LMUL = 8 according to more accurate cost model.

After this patch:

f:
	ble	a4,zero,.L5
.L3:
	vsetvli	a5,a4,e32,m8,ta,ma
	slli	a0,a5,2
	vle32.v	v16,0(a1)
	vadd.vi	v24,v16,15
	vadd.vv	v8,v24,v16
	vadd.vv	v0,v8,v16
	vse32.v	v0,0(a2)
	vadd.vv	v8,v8,v24
	vadd.vv	v8,v8,v16
	vse32.v	v8,0(a3)
	add	a1,a1,a0
	add	a2,a2,a0
	add	a3,a3,a0
	sub	a4,a4,a5
	bne	a4,zero,.L3
.L5:
	ret

Tested on both RV32 and RV64 no regression.

gcc/ChangeLog:

	* config/riscv/riscv-vector-costs.cc (variable_vectorized_p): Teach vi variant.

gcc/testsuite/ChangeLog:

	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul8-13.c: New test.