[Matrix] Implement += and -= for MatrixType.

Make sure CompLHSTy is set correctly for += and -= and matrix type
operands.

Bugzilla ticket is here https://bugs.llvm.org/show_bug.cgi?id=46164

Patch by Saurabh Jha <saurabh.jhaa@gmail.com>

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D98075