[LoopInterchange] Allow some loops with PHI nodes in the exit block.

We currently support LCSSA PHI nodes in the outer loop exit, if their
incoming values do not come from the outer loop latch or if the
outer loop latch has a single predecessor. In that case, the outer loop latch
will be executed only if the inner loop gets executed. If we have multiple
predecessors for the outer loop latch, it may be executed even if the inner
loop does not get executed.

This is a first step to support the case described in
https://bugs.llvm.org/show_bug.cgi?id=30472

Reviewers: efriedma, karthikthecool, mcrosier

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D43237

llvm-svn: 331037