Instead of the current approach of storing flat vectors in frame states (and possibly reusing the last vector in AST graph builder), this change list builds a tree for the values and tries to reuse the nodes for different frame states. At the moment, we only use this for the local variable part of frame state, but nothing prevents us from using this for all parts.
This change provides two new classes: one for creating the tree (StateValuesCache) and one for iterating the trees (StateValuesAccess).
BUG=
Review URL: https://codereview.chromium.org/1008213002
Cr-Commit-Position: refs/heads/master@{#27222}
