When the range ends just at the gap of a non-deferred block, the last
instruction the range covers is in the predecessor. If that predecessor is
a deferred block, before this CL, we would splinter the remainder of the
range all the way to the end. That leads to inefficient codegen, because
we still want a split inside the deferred block.

Also, opportunistically added a trace before we splinter, for better
diagnostics.

BUG= chromium:546416
LOG=N

Review URL: https://codereview.chromium.org/1412123009

Cr-Commit-Position: refs/heads/master@{#31529}