Idea: condense CFG

[nikomatsakis]

Currently we have an awful lot of points. However, it is rare that we need so many. I'm not sure of the exact conditions when we can compress two points, but it seems clear that we can. For example, if we have two points P and Q where

• P has one successor (Q)
• Q has one predecessor (P)
• The set of live regions at P is equal to set of live regions at Q
• There are no outlives relations at P or Q

then I think pretty clearly the subset relation would be the same during both of them.

We often know the first condition (one succ, one pred) just structurally: only the start of a basic block can have multiple preds, and only the mid point of a terminator can have multiple successors. The set of live regions is also easily approximated using the set of live variables, I imagine.