Add stack coallescing

[guillep]

Coalesce staged stack effects in different branches to allow compilation of static type predictions

• Add stack coalesce transformation
• review flush stack hoisting to hoist existing flush instructions too
• simplify management of exit basic blocks in CFG
• Add support to unspill stack slots

[guillep]

Ok, I finished the integration of the VM branch improvements. I'll now focus on integrating these and going over all your comments.