[Matrix] Implement + and - operators for MatrixType.

This patch implements the + and - binary operators for values of
MatrixType. It adds support for matrix +/- matrix, scalar +/- matrix and
matrix +/- scalar.

For the matrix, matrix case, the types must initially be structurally
equivalent. For the scalar,matrix variants, the element type of the
matrix must match the scalar type.

Reviewers: rjmccall, anemet, Bigcheese, rsmith, martong

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D76793