Refactor linear reg alloc to remember past assignments.

Proof of concept, might be that the performance is too bad.

Refactor linear reg alloc to remember past assignments.

We used to pick the first free register, which
is damn fast but produces fixup blocks on the regular.

Now we look for past assignments first. This means for
loops with blocks A, B, C if a variable is live in A & C
we will pick the right register (if available) in C.

This avoids some needless fixup blocks.