VECT: Add vec_mask_len_{load_lanes,store_lanes} patterns

This patch is add vec_mask_len_{load_lanes,store_stores} autovectorization patterns.

Here we want to support this following autovectorization:

void
foo (int8_t *__restrict a,
int8_t *__restrict b,
int8_t *__restrict cond,
int n)
{
  for (intptr_t i = 0; i < n; ++i)
    {
      if (cond[i])
        a[i] = b[i * 2] + b[i * 2 + 1];
    }
}

ARM SVE IR:

https://godbolt.org/z/cro1Eqc6a

  # loop_mask_60 = PHI <next_mask_82(4), max_mask_81(3)>
  ...
  mask__39.12_63 = vect__3.11_61 != { 0, ... };
  vec_mask_and_66 = loop_mask_60 & mask__39.12_63;
  ...
  vect_array.15 = .MASK_LOAD_LANES (_57, 8B, vec_mask_and_66);
  ...

For RVV, we would like to see IR:

  loop_len = SELECT_VL;
  ...
  mask__39.12_63 = vect__3.11_61 != { 0, ... };
  ...
  vect_array.15 = .MASK_LEN_LOAD_LANES (_57, 8B, mask__39.12_63, loop_len, bias);
  ...

Bootstrap and Regression on X86 passed.

Ok for trunk ?

gcc/ChangeLog:

	* doc/md.texi: Add vec_mask_len_{load_lanes,store_lanes} patterns.
	* internal-fn.cc (expand_partial_load_optab_fn): Ditto.
	(expand_partial_store_optab_fn): Ditto.
	* internal-fn.def (MASK_LEN_LOAD_LANES): Ditto.
	(MASK_LEN_STORE_LANES): Ditto.
	* optabs.def (OPTAB_CD): Ditto.