[mlir] [scf] fix crash when conversion from scf to control flow (#107221)

This patch fixed a crash when scf.parallel's region donesn't terminate
with reduce op. This can happend in dialect conversion.

Author: cxy-1993

mlir/lib/Conversion/SCFToControlFlow/SCFToControlFlow.cpp

@@ -482,7 +482,10 @@ LogicalResult
 ParallelLowering::matchAndRewrite(ParallelOp parallelOp,
                                   PatternRewriter &rewriter) const {
   Location loc = parallelOp.getLoc();
-  auto reductionOp = cast<ReduceOp>(parallelOp.getBody()->getTerminator());
+  auto reductionOp = dyn_cast<ReduceOp>(parallelOp.getBody()->getTerminator());
+  if (!reductionOp) {
+    return failure();
+  }
 
   // For a parallel loop, we essentially need to create an n-dimensional loop
   // nest. We do this by translating to scf.for ops and have those lowered in