Define patterns for the generation of MAC instructions

Created by: serkm

Hello Mr Krones,

I want to inform you, that I implemented support for the ISA extension of the CV32E40P in LLVM as part of my bachelor thesis at the University of Tübingen, and I am now working on integration of my work in your project.

You can find my implementation here: https://github.com/ekut-es/pulp-llvm My implementation includes definitions for the Instructions of the ISA extension, code generation for these instructions, generation of hardware loops and builtin functions for the direct use of these instructions.