[Bug tree-optimization/77445] [7 Regression] Performance drop after r239219 on coremark test

[law at redhat dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77445

--- Comment #22 from Jeffrey A. Law <law at redhat dot com> ---
In response to c#19.  Yes, a thread path which is cold except for a hot block
in the middle might be profitable to thread as it will sometimes expose path
specific redundancies/simplifications in the hot block.

Whether or not it does is mostly a function of the PHI nodes in the hot block. 
ISTM that these opportunities are most likely to arise if threading results in
a degenerate PHI in the hot block.  Which is something we ought to be able to
test for.

Not sure if that helps this testcase or not.