RISC-V: Teach liveness computation loop invariant shift amount

1). We not only have vashl_optab,vashr_optab,vlshr_optab which vectorize shift with vector shift amount,
that is, vectorization of 'a[i] >> x[i]', the shift amount is loop variant.
2). But also, we have ashl_optab, ashr_optab, lshr_optab which can vectorize shift with scalar shift amount,
that is, vectorization of 'a[i] >> x', the shift amount is loop invariant.

For the 2) case, we don't need to allocate a vector register group for shift amount.

So consider this following case:

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

Before this patch, we choose LMUL = 4, now after this patch, we can choose LMUL = 8:

f:
	ble	a5,zero,.L5
.L3:
	vsetvli	a0,a5,e32,m8,ta,ma
	slli	a6,a0,2
	vle32.v	v16,0(a1)
	vsra.vx	v24,v16,a4
	vmul.vv	v8,v24,v16
	vmul.vv	v0,v8,v16
	vse32.v	v0,0(a2)
	vmul.vv	v8,v8,v24
	vmul.vv	v8,v8,v16
	vsra.vx	v8,v8,a4
	vse32.v	v8,0(a3)
	add	a1,a1,a6
	add	a2,a2,a6
	add	a3,a3,a6
	sub	a5,a5,a0
	bne	a5,zero,.L3
.L5:
	ret

Tested on both RV32/RV64 no regression.  Ok for trunk ?

Note that we will apply same heuristic for vadd.vx, ... etc when the late-combine pass from
Richard Sandiford is committed (Since we need late combine pass to do vv->vx transformation for vadd).

gcc/ChangeLog:

	* config/riscv/riscv-vector-costs.cc (loop_invariant_op_p): New function.
	(variable_vectorized_p): Teach loop invariant.
	(has_unexpected_spills_p): Ditto.

gcc/testsuite/ChangeLog:

	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul4-12.c: New test.
	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul8-14.c: New test.