Disable jump threading into loop headers

Disable jump threading into loop headers

Consider this type of a loop:
    for (...) {
      ...
      if (...) continue;
      ...
    }
Normally, the "continue" would branch to the loop control code that
checks whether the loop should continue iterating and which contains
the (often) unique loop latch branch. In certain cases jump threading
can "thread" the inner branch directly to the loop header, creating
a second loop latch. Loop canonicalization would then transform this
loop into a loop nest. The problem with this is that in such a loop
nest neither loop is countable even if the original loop was. This
may inhibit subsequent loop optimizations and be detrimental to
performance.

Differential Revision: https://reviews.llvm.org/D36404

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@312664 91177308-0d34-0410-b5e6-96231b3b80d8