NFC. Improve isInnermostAffineForOp - drop unnecessary check

[bondhugula]

Summary

Rewrite isInnermostAffineForOp utility to make it more direct/efficient.
Drop unnecessary check.

[dcaballe]

LGTM, thanks!

mlir/lib/Dialect/Affine/Transforms/LoopUnroll.cpp

struct LoopUnroll : public AffineLoopUnrollBase<LoopUnroll> {
 
   void runOnOperation() override;
 
   /// Unroll this for op. Returns failure if nothing was done.
   LogicalResult runOnAffineForOp(AffineForOp forOp);
 };
 } // namespace
 
-/// Returns true if no other affine.for ops are nested within.
-static bool isInnermostAffineForOp(AffineForOp forOp) {
-  // Only for the innermost affine.for op's.
-  bool isInnermost = true;
-  forOp.walk([&](AffineForOp thisForOp) {
-    // Since this is a post order walk, we are able to conclude here.
-    isInnermost = (thisForOp == forOp);
+/// Returns true if no other affine.for ops are nested within `op`.
+static bool isInnermostAffineForOp(AffineForOp op) {
+  return !op.getBody()
+              ->walk([&](AffineForOp nestedForOp) {
                 return WalkResult::interrupt();
-  });
-  return isInnermost;
+              })
+              .wasInterrupted();
 }
 
 /// Gathers loops that have no affine.for's nested within.
 static void gatherInnermostLoops(FuncOp f,
                                  SmallVectorImpl<AffineForOp> &loops) {
   f.walk([&](AffineForOp forOp) {
     if (isInnermostAffineForOp(forOp))
       loops.push_back(forOp);