[LoopFusion] Forget loop and block dispositions after latch merge

Merging the latches of loops may affect the dispositions, so they
should be forgotten after the merge. This patch will fix the crash
due to using loop dispositions after forgetting them.

Author: 1997alireza

llvm/lib/Transforms/Scalar/LoopFuse.cpp

@@ -1796,14 +1796,15 @@ struct LoopFuser {
     // mergeLatch may remove the only block in FC1.
     SE.forgetLoop(FC1.L);
     SE.forgetLoop(FC0.L);
-    // Forget block dispositions as well, so that there are no dangling
-    // pointers to erased/free'ed blocks.
-    SE.forgetBlockAndLoopDispositions();
 
     // Move instructions from FC0.Latch to FC1.Latch.
     // Note: mergeLatch requires an updated DT.
     mergeLatch(FC0, FC1);
 
+    // Forget block dispositions as well, so that there are no dangling
+    // pointers to erased/free'ed blocks.
+    SE.forgetBlockAndLoopDispositions();
+
     // Merge the loops.
     SmallVector<BasicBlock *, 8> Blocks(FC1.L->blocks());
     for (BasicBlock *BB : Blocks) {
@@ -2092,14 +2093,15 @@ struct LoopFuser {
     // mergeLatch may remove the only block in FC1.
     SE.forgetLoop(FC1.L);
     SE.forgetLoop(FC0.L);
-    // Forget block dispositions as well, so that there are no dangling
-    // pointers to erased/free'ed blocks.
-    SE.forgetBlockAndLoopDispositions();
 
     // Move instructions from FC0.Latch to FC1.Latch.
     // Note: mergeLatch requires an updated DT.
     mergeLatch(FC0, FC1);
 
+    // Forget block dispositions as well, so that there are no dangling
+    // pointers to erased/free'ed blocks.
+    SE.forgetBlockAndLoopDispositions();
+
     // Merge the loops.
     SmallVector<BasicBlock *, 8> Blocks(FC1.L->blocks());
     for (BasicBlock *BB : Blocks) {