i386: Fix up avx_vpermilp_parallel [PR124138]

i386: Fix up avx_vpermilp_parallel [PR124138]

r16-3677 extended avx_vpermilp_parallel so that it handles also V8DImode
(among others) and broke this testcase.
For V8DFmode the bug existed there already since r0-127405 which extended
avx_vpermilp_parallel to handle V8DFmode.
For V16SImode/V16SFmode I think the code is correct, in that case it can
for V4SImode/V4SFmode choose any permutation it wants and the
V8S{I,F}mode and V16S{I,F}mode cases before FALLTHRU verify the upper lanes
match those lower ones.
For V[48]D[IF]mode the function uses different checking, where it checks
each pair of ipar separately:
    case E_V8DFmode:
    case E_V8DImode:
      /* In the 512-bit DFmode case, we can only move elements within
         a 128-bit lane.  First fill the second part of the mask,
         then fallthru.  */
      for (i = 4; i < 6; ++i)
        {
          if (ipar[i] < 4 || ipar[i] >= 6)
            return 0;
          mask |= (ipar[i] - 4) << i;
        }
      for (i = 6; i < 8; ++i)
        {
          if (ipar[i] < 6)
            return 0;
          mask |= (ipar[i] - 6) << i;
        }
      /* FALLTHRU */

    case E_V4DFmode:
    case E_V4DImode:
      /* In the 256-bit DFmode case, we can only move elements within
         a 128-bit lane.  */
      for (i = 0; i < 2; ++i)
        {
          if (ipar[i] >= 2)
            return 0;
          mask |= ipar[i] << i;
        }
      for (i = 2; i < 4; ++i)
        {
          if (ipar[i] < 2)
            return 0;
          mask |= (ipar[i] - 2) << i;
        }
      break;
so that it directly computes corresponding mask bit.
Earlier in the function it verified ipar[i] wasn't above
nelts (nor negative), so just checking if (ipar[i] >= 2) is ok, it can't
be negative, and just checking if (ipar[i] < 6) is also fine, because
it can never be 8 or above (because nelts is 8 in that case).
Though, the if (ipar[i] < 2) case check used to be correct only when
nelts could be only 4, when it can be 8 too, it will as in the following
testcase happily accept values in the [4, 7] range which shouldn't be
accepted (it can only handle [2, 3]).

Fixed thusly.

2026-02-17  Jakub Jelinek  <jakub@redhat.com>

	PR target/124138
	* config/i386/i386.cc (avx_vpermilp_parallel): Verify
	ipar[2] and ipar[3] aren't larger than 3.

	* gcc.dg/pr124138.c: New test.
	* gcc.target/i386/avx512f-pr124138-1.c: New test.
	* gcc.target/i386/avx512f-pr124138-2.c: New test.