Allocate memory for external snapshot paths in d8

R=marja@chromium.org
BUG=chromium:442804
LOG=n

Review URL: https://codereview.chromium.org/791713003

Cr-Commit-Position: refs/heads/master@{#25860}
[turbofan] Recursively reduce new inputs of changed nodes.

The GraphReducer should recurse into new inputs after a node was
changed. Most of the reducers already assume this behavior, and it
mostly worked by accident in many cases.

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/800073004

Cr-Commit-Position: refs/heads/master@{#25861}