[LoopInterchange] Check lcssa phis in the inner latch in scenarios of multi-level...

[LoopInterchange] Check lcssa phis in the inner latch in scenarios of multi-level nested loops

We already know that we need to check whether lcssa
phis are supported in inner loop exit block or in
outer loop exit block, and we have logic to check
them already. Presumably the inner loop latch does
not have lcssa phis and there is no code that deals
with lcssa phis in the inner loop latch. However,
that assumption is not true, when we have loops
with more than two-level nesting. This patch adds
checks for lcssa phis in the inner latch.

Reviewed By: Whitney

Differential Revision: https://reviews.llvm.org/D102300