[CodeLayout] Do not rebuild chains when opt for size

Author: ellishg

llvm/lib/CodeGen/MachineBlockPlacement.cpp

@@ -3558,14 +3558,16 @@ bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &MF) {
 
     if (BF.OptimizeFunction(MF, TII, MF.getSubtarget().getRegisterInfo(), MLI,
                             /*AfterPlacement=*/true)) {
-      // Redo the layout if tail merging creates/removes/moves blocks.
-      BlockToChain.clear();
-      ComputedEdges.clear();
       // Must redo the post-dominator tree if blocks were changed.
       if (MPDT)
         MPDT->recalculate(MF);
-      ChainAllocator.DestroyAll();
-      buildCFGChains();
+      if (!UseExtTspForSize) {
+        // Redo the layout if tail merging creates/removes/moves blocks.
+        BlockToChain.clear();
+        ComputedEdges.clear();
+        ChainAllocator.DestroyAll();
+        buildCFGChains();
+      }
     }
   }