[IRCE] Add a missing invariant check

[IRCE] Add a missing invariant check

Currently IRCE relies on the loops it transforms to be (semantically) of
the form:

  for (i = START; i < END; i++)
    ...

or

  for (i = START; i > END; i--)
    ...

However, we were not verifying the presence of the START < END entry
check (i.e. check before the first iteration).  We were only verifying
that the backedge was guarded by (i + 1) < END.

Usually this would work "fine" since (especially in Java) most loops do
actually have the START < END check, but of course that is not
guaranteed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@294375 91177308-0d34-0410-b5e6-96231b3b80d8