This routine is not for general use.  It exists solely to avoid
 over-optimization in DISNAN.

 DLAISNAN checks for NaNs by comparing its two arguments for
 inequality.  NaN is the only floating-point value where NaN != NaN
 returns .TRUE.  To check for NaNs, pass the same variable as both
 arguments.

 A compiler must assume that the two arguments are
 not the same variable, and the test will not be optimized away.
 Interprocedural or whole-program optimization may delete this
 test.  The ISNAN functions will be replaced by the correct
 Fortran 03 intrinsic once the intrinsic is widely available.

          DIN1 is DOUBLE PRECISION

          DIN2 is DOUBLE PRECISION
          Two numbers to compare for inequality.