VECT: Add mask_len_fold_left_plus for in-order floating-point reduction

VECT: Add mask_len_fold_left_plus for in-order floating-point reduction

Hi, Richard and Richi.

This patch adds mask_len_fold_left_plus pattern to support in-order floating-point
reduction for target support len loop control.

Consider this following case:
double
foo2 (double *__restrict a,
     double init,
     int *__restrict cond,
     int n)
{
    for (int i = 0; i < n; i++)
      if (cond[i])
        init += a[i];
    return init;
}

ARM SVE:

...
vec_mask_and_60 = loop_mask_54 & mask__23.33_57;
vect__ifc__35.37_64 = .VCOND_MASK (vec_mask_and_60, vect__8.36_61, { 0.0, ... });
_36 = .MASK_FOLD_LEFT_PLUS (init_20, vect__ifc__35.37_64, loop_mask_54);
...

For RVV, we want to see:
...
_36 = .MASK_LEN_FOLD_LEFT_PLUS (init_20, vect__ifc__35.37_64, control_mask, loop_len, bias);
...

gcc/ChangeLog:

	* doc/md.texi: Add mask_len_fold_left_plus.
	* internal-fn.cc (mask_len_fold_left_direct): Ditto.
	(expand_mask_len_fold_left_optab_fn): Ditto.
	(direct_mask_len_fold_left_optab_supported_p): Ditto.
	* internal-fn.def (MASK_LEN_FOLD_LEFT_PLUS): Ditto.
	* optabs.def (OPTAB_D): Ditto.