[SimpleLoopUnswitch] Fix SCEV invalidation issue

This patch is making sure that we use getTopMostExitingLoop when
finding out which loops to forget, when dealing with
unswitchNontrivialInvariants and unswitchTrivialSwitch. It seems
to at least be needed for unswitchNontrivialInvariants as detected
by the included test case.

Note that unswitchTrivialBranch already used getTopMostExitingLoop.
This was done in commit 4a9cde5a. The commit
message in that commit says "If the patch makes sense, I will also
update those places to a similar approach ...", referring to these
functions mentioned above. As far as I can tell that never happened,
but this is an attempt to finally fix that.

Fixes https://github.com/llvm/llvm-project/issues/61080

Differential Revision: https://reviews.llvm.org/D147058