This PR was caused by mismatched expectations between
vectorizable_comparison and SLP.  We had a "<" comparison
between two booleans that were leaves of the SLP tree, so
vectorizable_comparison fell back on:

  /* Invariant comparison.  */
  if (!vectype)
    {
      vectype = get_vectype_for_scalar_type (vinfo, TREE_TYPE (rhs1),
                                             slp_node);
      if (maybe_ne (TYPE_VECTOR_SUBPARTS (vectype), nunits))
        return false;
    }

rhs1 and rhs2 were *unsigned* boolean types, so we got back a vector
of unsigned integers.  This in itself was OK, and meant that "<"
worked as expected without the need for the boolean fix-ups:

  /* Boolean values may have another representation in vectors
     and therefore we prefer bit operations over comparison for
     them (which also works for scalar masks).  We store opcodes
     to use in bitop1 and bitop2.  Statement is vectorized as
       BITOP2 (rhs1 BITOP1 rhs2) or
       rhs1 BITOP2 (BITOP1 rhs2)
     depending on bitop1 and bitop2 arity.  */
  bool swap_p = false;
  if (VECTOR_BOOLEAN_TYPE_P (vectype))
    {

However, vectorizable_comparison then used vect_get_slp_defs to get
the actual operands.  The request went to vect_get_constant_vectors,
which also has logic to calculate the vector type.  The problem was
that this type was different from the one chosen above:

  if (VECT_SCALAR_BOOLEAN_TYPE_P (TREE_TYPE (op))
      && vect_mask_constant_operand_p (stmt_vinfo))
    vector_type = truth_type_for (stmt_vectype);
  else
    vector_type = get_vectype_for_scalar_type (vinfo, TREE_TYPE (op), op_node);

So the function gave back a vector of mask types, which here are vectors
of *signed* booleans.  This meant that "<" gave:

  true (-1) < false (0)

and so the boolean fixup above was needed after all.

Fixed by making vectorizable_comparison also pick a mask type in
this case.

2020-04-23  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR tree-optimization/94727
	* tree-vect-stmts.c (vectorizable_comparison): Use mask_type when
	comparing invariant scalar booleans.

gcc/testsuite/
	PR tree-optimization/94727
	* gcc.dg/vect/pr94727.c: New test.