[Matrix] Add support for matrix-by-scalar division.

This patch extends the matrix spec to allow matrix-by-scalar division.

Originally support for `/` was left out to avoid ambiguity for the
matrix-matrix version of `/`, which could either be elementwise or
specified as matrix multiplication M1 * (1/M2).

For the matrix-scalar version, no ambiguity exists; `*` is also
an elementwise operation in that case. Matrix-by-scalar division
is commonly supported by systems including Matlab, Mathematica
or NumPy.

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D97857