The notion of an unreachable environment is useful for a recursive
descent iteration (e.g. over an AST) where nodes are created on the
ascent path as well. For a flat iteration (e.g. over bytecode stream)
environments become unreachable at the end of a visitation function.
Hence any unreachable path can be represented by nulling the tracked
environment completely. This further reduces the number of redundant
nodes being created.

R=oth@chromium.org

Review URL: https://codereview.chromium.org/1650483003

Cr-Commit-Position: refs/heads/master@{#33639}