Loop coalescing: fix pointer chainsing in use-chain traversal

In the replaceAllUsesExcept utility function called from loop coalescing the
iteration over the use-chain is incorrect. The use list nodes (IROperands) have
next/prev links, and bluntly resetting the use would make the loop to continue
on uses of the value that was replaced instead of the original one. As a
result, it could miss the existing uses and update the wrong ones. Make sure we
increment the iterator before updating the use in the loop body.

Reported-by: Uday Bondhugula <uday@polymagelabs.com>

Closes #291.

PiperOrigin-RevId: 283754195
Change-Id: Ia9478e50544b2cd30b42e18262e3038009cf05b3

third_party/mlir/lib/Transforms/Utils/LoopUtils.cpp

@@ -979,7 +979,7 @@ TileLoops mlir::extractFixedOuterLoops(loop::ForOp rootForOp,
 static void
 replaceAllUsesExcept(Value *orig, Value *replacement,
                      const SmallPtrSetImpl<Operation *> &exceptions) {
-  for (auto &use : orig->getUses()) {
+  for (auto &use : llvm::make_early_inc_range(orig->getUses())) {
     if (exceptions.count(use.getOwner()) == 0)
       use.set(replacement);
   }