[NFC][MLIR] Fix: alloca promotion for AllocationOpInterface (llvm#97672)

The std::optional returned by buildPromotedAlloc was directly
dereferenced and assumed to be non-null, even though the documentation
for AllocationOpInterface indicates that std::nullopt is a legal value
if buffer stack promotion is not supported (and is the default value
supplied by the TableGen interface file). This patch removes the direct
dereference so that the optional can be null-checked prior to use.

Co-authored-by: Nikhil Kalra <[email protected]>

Author: nikalra

mlir/lib/Dialect/Bufferization/Transforms/BufferOptimizations.cpp

@@ -397,12 +397,12 @@ class BufferPlacementPromotion : BufferPlacementTransformationBase {
       OpBuilder builder(startOperation);
       Operation *allocOp = alloc.getDefiningOp();
       if (auto allocInterface = dyn_cast<AllocationOpInterface>(allocOp)) {
-        Operation *alloca =
-            allocInterface.buildPromotedAlloc(builder, alloc).value();
+        std::optional<Operation *> alloca =
+            allocInterface.buildPromotedAlloc(builder, alloc);
         if (!alloca)
           continue;
         // Replace the original alloc by a newly created alloca.
-        allocOp->replaceAllUsesWith(alloca);
+        allocOp->replaceAllUsesWith(alloca.value());
         allocOp->erase();
       }
     }