Merging 182989: Fix ScalarEvolution::ComputeExitLimitFromCond for 'or' conditions.

    Fixes PR16130 - clang produces incorrect code with loop/expression at -O2.

    This is a 2+ year old bug that's now holding up the release. It's a
    case where we knowingly made aggressive assumptions about undefined
    behavior. These assumptions are wrong when SCEV is computing a
    subexpression that does not directly control the branch. With this
    fix, we avoid making assumptions in those cases but still optimize the
    common case. SCEV's trip count computation for exits controlled by
    'or' expressions is now analagous to the trip count computation for
    loops with multiple exits. I had already fixed the multiple exit case
    to be conservative.