Minor performance improments

Mostly useful as lesson learned.

1) The double precision constant forces the compiler to convert
   the single precision variable to double, then do the multiplication
   in double and then convert back. Using the single precsion
   constant in double reduces the accuracy (the calculation is still done
   double but the constant has only single precision).
2) Using a temporary array instead of a temporary scalar causes ICC14 to
   generate an extra store.

Change-Id: Ib320ac2ae4ff80ce48277544abff468c483cc83a