feature[next]: Extend Single Static Assignment (SSA) pass to support if statements

[tehrengruber]

This PR extends the SSA pass (operating on the Python AST) to support if statements such that every variable is assigned only once per branch, e.g. this

if True:
    a = 1
else:
    a = 2
    if True:
        a = 3
    a = 4
a = 5

is transformed into

if True:
    a__0 = 1
    a__2 = a__0
else:
    a__0 = 2
    if True:
        a__1 = 3
    else:
        a__1 = a__0
    a__2 = 4
a__3 = 5

Most of the code was written by @BenWeber42 reviewed by me. See #972 for the discussion. The PR is a prerequisite for #1079 and was spun off from there to ease review.

[egparedes]

The algorithm looks good but the implementation looks too verbose to me and unnecessarily hard to read. I recommend to simplify the code before approving the Pr.

[tehrengruber]

Ready for another round.

[egparedes]

It looks almost ready to go. Just some comments about missing type annotations and docstrings.

[egparedes]

Yes, I understood the same core idea but don't really get the details. I think we should understand what the comment really means and rephrase it in a more comprehensible way before merging the PR, otherwise is useless.

[egparedes]

ILGTM.