[mlir][mesh] Add null check for dyn_cast to prevent crash

This PR adds a null check for dyn_cast to prevent crash, and use `isa` instead `dyn_cast` to make code clean.

Author: CoTinker

mlir/include/mlir/Dialect/Mesh/Transforms/Simplifications.h

@@ -62,9 +62,11 @@ void populateAllReduceEndomorphismSimplificationPatterns(
   auto isEndomorphismOp = [reduction](Operation *op,
                                       std::optional<Operation *> referenceOp) {
     auto allReduceOp = llvm::dyn_cast<AllReduceOp>(op);
+    if (!allReduceOp)
+      return false;
     auto inType = cast<ShapedType>(allReduceOp.getInput().getType());
     auto outType = cast<ShapedType>(allReduceOp.getResult().getType());
-    if (!allReduceOp || inType.getElementType() != outType.getElementType() ||
+    if (inType.getElementType() != outType.getElementType() ||
         allReduceOp.getReduction() != reduction) {
       return false;
     }
@@ -87,9 +89,7 @@ void populateAllReduceEndomorphismSimplificationPatterns(
     return refAllReduceOp->getAttrs() == allReduceOp->getAttrs() &&
            inType.getElementType() == refType.getElementType();
   };
-  auto isAlgebraicOp = [](Operation *op) {
-    return static_cast<bool>(llvm::dyn_cast<AlgebraicOp>(op));
-  };
+  auto isAlgebraicOp = [](Operation *op) { return isa<AlgebraicOp>(op); };
 
   using ConcreteEndomorphismSimplification = EndomorphismSimplification<
       std::decay_t<decltype(getEndomorphismOpOperand)>,