This catches malformed code like the following example:

<expr>
<block begin>
<expr>
<binop>
<end>

Which is illegal because the inputs to the binop cross the block boundary.

R=rossberg@chromium.org,ahaas@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/1961853002
Cr-Commit-Position: refs/heads/master@{#36103}