[Matrix] Add info about number of operations to remarks.

This patch updates the remark to also include a summary of the number of
vector operations generated for each matrix expression.

Reviewers: anemet, Gerolf, thegameg, hfinkel, andrew.w.kaylor, LuoYuanke

Reviewed By: anemet

Differential Revision: https://reviews.llvm.org/D72480