SLAS2  computes the singular values of the 2-by-2 matrix
    [  F   G  ]
    [  0   H  ].
 On return, SSMIN is the smaller singular value and SSMAX is the
 larger singular value.

          F is REAL
          The (1,1) element of the 2-by-2 matrix.

          G is REAL
          The (1,2) element of the 2-by-2 matrix.

          H is REAL
          The (2,2) element of the 2-by-2 matrix.

          SSMIN is REAL
          The smaller singular value.

          SSMAX is REAL
          The larger singular value.

  Barring over/underflow, all output quantities are correct to within
  a few units in the last place (ulps), even in the absence of a guard
  digit in addition/subtraction.

  In IEEE arithmetic, the code works correctly if one matrix element is
  infinite.

  Overflow will not occur unless the largest singular value itself
  overflows, or is within a few ulps of overflow. (On machines with
  partial overflow, like the Cray, overflow may occur if the largest
  singular value is within a factor of 2 of overflow.)

  Underflow is harmless if underflow is gradual. Otherwise, results
  may correspond to a matrix modified by perturbations of size near
  the underflow threshold.