veclower: Fix up vec_shl matching of VEC_PERM_EXPR [PR100239]

veclower: Fix up vec_shl matching of VEC_PERM_EXPR [PR100239]

The following testcase ICEs at -O0, because lower_vec_perm sees the
  _1 = { 0, 0, 0, 0, 0, 0, 0, 0 };
  _2 = VEC_COND_EXPR <_1, { -1, -1, -1, -1, -1, -1, -1, -1 }, { 0, 0, 0, 0, 0, 0, 0, 0 }>;
  _3 = { 6, 0, 0, 0, 0, 0, 0, 0 };
  _4 = VEC_PERM_EXPR <{ 0, 0, 0, 0, 0, 0, 0, 0 }, _2, _3>;
and as the ISA is SSE2, there is no support for the particular permutation
nor for variable mask permutation.  But, the code to match vec_shl matches
it, because the permutation has the first operand a zero vector and the
mask picks all elements randomly from that vector.
So, in the end that isn't a vec_shl, but the permutation could be in theory
optimized into the first argument.  As we keep it as is, it will fail
during expansion though, because that for vec_shl correctly requires that
it actually is a shift:
      unsigned firstidx = 0;
      for (unsigned int i = 0; i < nelt; i++)
        {
          if (known_eq (sel[i], nelt))
            {
              if (i == 0 || firstidx)
                return NULL_RTX;
              firstidx = i;
            }
          else if (firstidx
                   ? maybe_ne (sel[i], nelt + i - firstidx)
                   : maybe_ge (sel[i], nelt))
            return NULL_RTX;
        }

      if (firstidx == 0)
        return NULL_RTX;
      first = firstidx;
The if (firstidx == 0) return NULL; is what is missing a counterpart
on the lower_vec_perm side.
As with optimize != 0 we fold it in other spots, I think it is not needed
to optimize this cornercase in lower_vec_perm (which would mean we'd need
to recurse on the newly created _4 = { 0, 0, 0, 0, 0, 0, 0, 0 };
whether it is supported or not).

2021-04-27  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/100239
	* tree-vect-generic.c (lower_vec_perm): Don't accept constant
	permutations with all indices from the first zero element as vec_shl.

	* gcc.dg/pr100239.c: New test.