[InstCombine] Track inserted instructions when lowering objectsize

The inserted instructions can usually be simplified. Make sure this
happens in the same InstCombine iteration by adding them to the
worklist.

We happen to get some better optimization in two cases, but this is
just a lucky accident. https://github.com/llvm/llvm-project/issues/63472
tracks implementing a fold for that case.

This doesn't track all inserted instructions yet, for that we would
also have to include those created by ObjectSizeOffsetEvaluator.