VECT: Support CALL vectorization for COND_LEN_*

VECT: Support CALL vectorization for COND_LEN_*

Hi, Richard and Richi.

Base on the suggestions from Richard:
https://gcc.gnu.org/pipermail/gcc-patches/2023-July/625396.html

This patch choose (1) approach that Richard provided, meaning:

RVV implements cond_* optabs as expanders.  RVV therefore supports
both IFN_COND_ADD and IFN_COND_LEN_ADD.  No dummy length arguments
are needed at the gimple level.

Such approach can make codes much cleaner and reasonable.

Consider this following case:
void foo (float * __restrict a, float * __restrict b, int * __restrict cond, int n)
{
  for (int i = 0; i < n; i++)
    if (cond[i])
      a[i] = b[i] + a[i];
}

Output of RISC-V (32-bits) gcc (trunk) (Compiler #3)
<source>:5:21: missed: couldn't vectorize loop
<source>:5:21: missed: not vectorized: control flow in loop.

ARM SVE:

...
mask__27.10_51 = vect__4.9_49 != { 0, ... };
...
vec_mask_and_55 = loop_mask_49 & mask__27.10_51;
...
vect__9.17_62 = .COND_ADD (vec_mask_and_55, vect__6.13_56, vect__8.16_60, vect__6.13_56);

For RVV, we want IR as follows:

...
_68 = .SELECT_VL (ivtmp_66, POLY_INT_CST [4, 4]);
...
mask__27.10_51 = vect__4.9_49 != { 0, ... };
...
vect__9.17_60 = .COND_LEN_ADD (mask__27.10_51, vect__6.13_55, vect__8.16_59, vect__6.13_55, _68, 0);
...

Both len and mask of COND_LEN_ADD are real not dummy.

This patch has been fully tested in RISC-V port with supporting both COND_* and COND_LEN_*.

And also, Bootstrap and Regression on X86 passed.

OK for trunk?

gcc/ChangeLog:

	* internal-fn.cc (get_len_internal_fn): New function.
	(DEF_INTERNAL_COND_FN): Ditto.
	(DEF_INTERNAL_SIGNED_COND_FN): Ditto.
	* internal-fn.h (get_len_internal_fn): Ditto.
	* tree-vect-stmts.cc (vectorizable_call): Add CALL auto-vectorization.