[mlir]use correct iterator when eraseOp (llvm#83444)

llvm#66771 introduce `llvm::post_order(&r.front())` which is equal to
`r.front().getSuccessor(...)`.
It will visit the succ block of current block. But actually here need to
visit all block of region in reverse order.
Fixes: llvm#77420.

Author: HerrCai0907

mlir/lib/IR/PatternMatch.cpp

@@ -229,7 +229,10 @@ void RewriterBase::eraseOp(Operation *op) {
       // until the region is empty. (The block graph could be disconnected.)
       while (!r.empty()) {
         SmallVector<Block *> erasedBlocks;
-        for (Block *b : llvm::post_order(&r.front())) {
+        // Some blocks may have invalid successor, use a set including nullptr
+        // to avoid null pointer.
+        llvm::SmallPtrSet<Block *, 4> visited{nullptr};
+        for (Block *b : llvm::post_order_ext(&r.front(), visited)) {
           // Visit ops in reverse order.
           for (Operation &op :
                llvm::make_early_inc_range(ReverseIterator::makeIterable(*b)))

mlir/test/Transforms/gh-77420.mlir

@@ -0,0 +1,21 @@
+// RUN: mlir-opt --canonicalize %s | FileCheck %s
+
+
+module {
+
+// CHECK:       func.func @f() {
+// CHECK-NEXT:    return
+// CHECK-NEXT:  }
+  func.func @f() {
+    return
+  ^bb1:  // no predecessors
+    omp.parallel   {
+      %0 = llvm.intr.stacksave : !llvm.ptr
+      llvm.br ^bb1
+    ^bb1:  // pred: ^bb0
+      omp.terminator
+    }
+    return
+  }
+
+}