[llvm] [Codegen] (NFC) Faster algorithm for MachineBlockPlacement (PR #91843)

[William Junda Huang via llvm-commits]

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@@ -3158,9 +3158,9 @@ bool MachineBlockPlacement::maybeTailDuplicateBlock(
 
         // Handle the filter set
         if (BlockFilter) {
-          BlockFilter->remove(RemBB);
           if (*PrevUnplacedBlockInFilterIt == RemBB)
             PrevUnplacedBlockInFilterIt++;
+          BlockFilter->remove(RemBB);
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huangjd wrote:

BlockFilter is a SmallSetVector (which is backed by a small vector) and iterators are tied to the underlying vector (instead of the set), and removing an element shifts following elements one position forward without reallocation so the iterator should be valid (and pointing to the following element if remove happens at or before PrevUnplacedBlockInFilterIt)

https://github.com/llvm/llvm-project/pull/91843