[SimpleLoopUnswitch] Fix exponential unswitch

When unswitching via invariant condition injection, we currently
mark the condition in the old loop, so that it does not get
unswitched again. However, if there are multiple branches for
which conditions can be injected, then we can do that for both
the old and new loop. This means that the number of unswitches
increases exponentially.

Change the handling to be more similar to partial unswitching,
where we instead mark the whole loop, rather than a single
condition. This means that we will only generate a linear number
of loops.

TBH I think even that is still highly undesirable, and we should
probably be unswitching all candidates at the same time, so that
we end up with only two loops. But at least this mitigates the
worst case.

The test case is a reduced variant that generates 1700 lines of IR
without this patch and 290 with it.

Fixes https://github.com/llvm/llvm-project/issues/66868.