[SCEV] Fix isKnownPredicate

IsKnownPredicate is updated to implement the following algorithm
proposed by @sanjoy and @mkazantsev :
isKnownPredicate(Pred, LHS, RHS) {
  Collect set S all loops on which either LHS or RHS depend.
  If S is non-empty
    a. Let PD be the element of S which is dominated by all other elements of S
    b. Let E(LHS) be value of LHS on entry of PD.
       To get E(LHS), we should just take LHS and replace all AddRecs that
       are attached to PD on with their entry values.
       Define E(RHS) in the same way.
    c. Let B(LHS) be value of L on backedge of PD.
       To get B(LHS), we should just take LHS and replace all AddRecs that
       are attached to PD on with their backedge values.
       Define B(RHS) in the same way.
    d. Note that E(LHS) and E(RHS) are automatically available on entry of PD,
       so we can assert on that.
    e. Return true if isLoopEntryGuardedByCond(Pred, E(LHS), E(RHS)) &&
                      isLoopBackedgeGuardedByCond(Pred, B(LHS), B(RHS))
Return true if Pred, L, R is known from ranges, splitting etc.
}
This is follow-up for https://reviews.llvm.org/D42417.

Reviewers: sanjoy, mkazantsev, reames
Reviewed By: sanjoy, mkazantsev
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D43507

llvm-svn: 327362