Fix Eigen-related compilation error when using -mavx2. (#3792)

Since updating to use a newer version of Eigen [1], compilation with
-mavx2 causes an error caused by an undefined type. Eigen removed
the type `scalar_multipl2_op` [2] so `scalar_product_op` should now
be used.

This commit replaces uses of `scalar_multiple2_op` similarly to how
[2] did this.

[1] See chage 127264575, commit 10211a6c.
[2] See https://bitbucket.org/eigen/eigen/commits/03556a17eb548275bc9404d7cda8303ff6ca5c13