[LoopInterchange] Check phis in all subloops

This is the bugfix to the miscompile mentioned in
https://reviews.llvm.org/D132055#3814831. The IR
that reproduced the bug is added as the test case in
this patch.

What this patch does is that, during legality phase
instead of checking the phi nodes only in `InnerLoop`
and `OuterLoop`, we check phi nodes in all subloops
of the `OuterLoop`. Suppose if the loop nest is triply
nested, and `InnerLoop` and `OuterLoop` is the middle
loop and the outermost loop respectively, we'll check
phi nodes in the innermost loop as well, in addition to
the ones in the middle and outermost loops.

Reviewed By: Meinersbur, #loopoptwg

Differential Revision: https://reviews.llvm.org/D134930