[SimpleLoopUnswitch] Reland "Generalize the notion of trivial unswitching" (#204934)

In the original PR for this feature, the third parameter of
`MemorySSAUpdater::applyUpdates` was not set to `true`. This
caused `DT` to be invalid, resulting in the failures. This PR fixes the
issue and provides an additional testcase.

Also in the original commit had three asserts to confirm
`!ModifiedBranch ` when we bail out. One of these is redundant because
it is enclosed in an `if` statement checking the same condition. This PR removes
the redundant `assert` too.

GitOrigin-RevId: e8b663ba31d4eed75a6cbc758dbc9f56589c9239