This is fix for
[187902](https://github.com/llvm/llvm-project/issues/187902), where
`LoopInfo` is not in a valid state at the beginning of `ScalarEvolution::createSCEVIter`.
The reason for the bug is that, `mergeLatch()` is called at a place
where control flow and dominator trees have been updated but `LoopInfo`
has not completed the update yet. `mergeLatch()` calls into
`ScalarEvolution` that uses `LoopInfo`, where out-of-date `LoopInfo` would
result in crash or unpredictable results.
This patch moves `mergeLatch()` to the place where `LoopInfo` has
completed its update and hence is in a valid state.
fbe6d79465