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

 SLAISNAN 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.

          SIN1 is REAL

          SIN2 is REAL
          Two numbers to compare for inequality.