[LoopDeletion] Forget block and loop dispositions after deleting loop.

[fhahn]

Summary

After deleting a loop, the block and loop dispositions need to be
cleared. As we don't know which SCEVs in the loop/blocks may be
impacted, completely clear the cache. This should also fix some cases
where deleted loops remained in the LoopDispositions cache.

This fixes a verification failure surfaced by D134531.

[mkazantsev]

LG

[fhahn]

Rebase on current main.

llvm/lib/Transforms/Utils/LoopUtils.cpp

void llvm::deleteDeadLoop(Loop *L, DominatorTree *DT, ScalarEvolution *SE,
   // branch from the preheader to go to the single exit block.
   //
   // Because we're deleting a large chunk of code at once, the sequence in which
   // we remove things is very important to avoid invalidation issues.
 
   // Tell ScalarEvolution that the loop is deleted. Do this before
   // deleting the loop so that ScalarEvolution can look at the loop
   // to determine what it needs to clean up.
-  if (SE)
+  if (SE) {
     SE->forgetLoop(L);
+    SE->forgetBlockAndLoopDispositions();
+  }
 
   Instruction *OldTerm = Preheader->getTerminator();
   assert(!OldTerm->mayHaveSideEffects() &&
          "Preheader must end with a side-effect-free terminator");
   assert(OldTerm->getNumSuccessors() == 1 &&
          "Preheader must have a single successor");
   // Connect the preheader to the exit block. Keep the old edge to the header
   // around to perform the dominator tree update in two separate steps