freeze heuristics, slight change to tree gen

Yi Linrequested to merge
defuse into master
  • a freeze heuristic that prioritise freezing less frequently used nodes
  • in tree generation, always put instructions that yield int<1> as child
  • use P<TreeNode instead of Box<TreeNode> for block body
  • def-use pass records definitions of each SSA variables (not used for now)

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