Incremental SSA using Braun's method implemented

[dibyendumajumdar]

The optvm compiler now has an option to generate SSA IR straight from AST, using Braun's method.

https://dl.acm.org/doi/10.1007/978-3-642-37051-9_6

This is a compiler option, alternative pipeline uses the classic Cytron / Briggs SSA construction.