Vect: Support loop len in vectorizable early exit

Vect: Support loop len in vectorizable early exit

This patch adds early break auto-vectorization support for target which
use length on partial vectorization.  Consider this following example:

unsigned vect_a[802];
unsigned vect_b[802];

void test (unsigned x, int n)
{
  for (int i = 0; i < n; i++)
  {
    vect_b[i] = x + i;

    if (vect_a[i] > x)
      break;

    vect_a[i] = x;
  }
}

We use VCOND_MASK_LEN to simulate the generate (mask && i < len + bias).
And then the IR of RVV looks like below:

  ...
  _87 = .SELECT_VL (ivtmp_85, POLY_INT_CST [32, 32]);
  _55 = (int) _87;
  ...
  mask_patt_6.13_69 = vect_cst__62 < vect__3.12_67;
  vec_len_mask_72 = .VCOND_MASK_LEN (mask_patt_6.13_69, { -1, ... }, \
    {0, ... }, _87, 0);
  if (vec_len_mask_72 != { 0, ... })
    goto <bb 6>; [5.50%]
  else
    goto <bb 7>; [94.50%]

The below tests are passed for this patch:
1. The riscv fully regression tests.
2. The x86 bootstrap tests.
3. The x86 fully regression tests.

gcc/ChangeLog:

	* tree-vect-loop.cc (vect_gen_loop_len_mask): New func to gen
	the loop len mask.
	* tree-vect-stmts.cc (vectorizable_early_exit): Invoke the
	vect_gen_loop_len_mask for 1 or more stmt(s).
	* tree-vectorizer.h (vect_gen_loop_len_mask): New func decl
	for vect_gen_loop_len_mask.

Signed-off-by: Pan Li <pan2.li@intel.com>