🐛 do not invalidate iterator in block collection (#808)

## Description

This fixes a small bug observed as part of #803, which only surfaced on
Windows in Debug mode when compiling using Clang 🤯
Part of the block collection code in the circuit optimizer would
invalidate an iterator due to an update of the respective container
being iterated over within the loop.

## Checklist:

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- [x] The pull request only contains commits that are related to it.
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project's style guidelines.

Signed-off-by: Lukas Burgholzer <[email protected]>

Author: burgholzer

src/circuit_optimizer/CircuitOptimizer.cpp

@@ -1414,7 +1414,10 @@ struct DSU {
     } else {
       *currentBlockInCircuit[block] = std::move(compoundOp);
     }
-    for (auto i : bitBlocks[block]) {
+    // need to make a copy here because otherwise the updates in the loop might
+    // invalidate the iterator
+    const auto blockBits = bitBlocks[block];
+    for (auto i : blockBits) {
       parent[i] = i;
       bitBlocks[i] = {i};
       currentBlockInCircuit[i] = nullptr;