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

[William Junda Huang via llvm-commits]

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

Code is updated.  On a side note why couldn't SmallVector guarantee iterator validity after element removal, given that LLVM is in control of its implementation? 

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