[CodeGen] Restore MachineBlockPlacement block ordering (#99351)

PR #91843 changed the algorithm used to find the next unplaced block so
that it iterates through the blocks in BlockFilter instead of iterating
through the blocks in the function and checking if they are in the block
filter. Unfortunately this sometimes results in a different block
ordering being chosen, as the order of blocks in BlockFilter comes from
the order in MachineLoopInfo, and in some cases this differs from the
order they are in the function. This can also give an end result that
has worse performance.

Fix this by making collectLoopBlockSet place blocks in its output in the
order that they are in the function.

Author: john-brawn-arm

llvm/lib/CodeGen/MachineBlockPlacement.cpp

@@ -2596,7 +2596,15 @@ void MachineBlockPlacement::rotateLoopWithProfile(
 /// otherwise, collect all blocks in the loop.
 MachineBlockPlacement::BlockFilterSet
 MachineBlockPlacement::collectLoopBlockSet(const MachineLoop &L) {
-  BlockFilterSet LoopBlockSet;
+  // Collect the blocks in a set ordered by block number, as this gives the same
+  // order as they appear in the function.
+  struct MBBCompare {
+    bool operator()(const MachineBasicBlock *X,
+                    const MachineBasicBlock *Y) const {
+      return X->getNumber() < Y->getNumber();
+    }
+  };
+  std::set<const MachineBasicBlock *, MBBCompare> LoopBlockSet;
 
   // Filter cold blocks off from LoopBlockSet when profile data is available.
   // Collect the sum of frequencies of incoming edges to the loop header from
@@ -2627,7 +2635,11 @@ MachineBlockPlacement::collectLoopBlockSet(const MachineLoop &L) {
   } else
     LoopBlockSet.insert(L.block_begin(), L.block_end());
 
-  return LoopBlockSet;
+  // Copy the blocks into a BlockFilterSet, as iterating it is faster than
+  // std::set. We will only remove blocks and never insert them, which will
+  // preserve the ordering.
+  BlockFilterSet Ret(LoopBlockSet.begin(), LoopBlockSet.end());
+  return Ret;
 }
 
 /// Forms basic block chains from the natural loop structures.