Validate that the transition from deferred to hot happens through a
deferred block with one successor. This is needed for frame elision: if
we need to deconstruct the frame on the deferred path, this extra block
offers that location.

A precondition for this validation is that the sequence is in split edge
form.

BUG=

Review URL: https://codereview.chromium.org/1811213003

Cr-Commit-Position: refs/heads/master@{#35015}