[mlir] fix crash when call a function decl

[python3kgae]

Summary

Check region before use it.
Fixes #60215 https://github.com/llvm/llvm-project/issues/60215

mlir/lib/Analysis/DataFlow/SparseAnalysis.cpp

void AbstractSparseBackwardDataFlowAnalysis::visitOperation(Operation *op) {
   }
 
   // For function calls, connect the arguments of the entry blocks
   // to the operands of the call op.
   if (auto call = dyn_cast<CallOpInterface>(op)) {
     Operation *callableOp = call.resolveCallable(&symbolTable);
     if (auto callable = dyn_cast_or_null<CallableOpInterface>(callableOp)) {
       Region *region = callable.getCallableRegion();
-      if (!region->empty()) {
+      if (region && !region->empty()) {
         Block &block = region->front();
         for (auto [blockArg, operand] :
              llvm::zip(block.getArguments(), operandLattices)) {
           meet(operand, *getLatticeElementFor(op, blockArg));
         }
       }
       return;
     }

mlir/test/Analysis/DataFlow/test-written-to.mlir

default {
     %6 = arith.constant {tag="default_a"} 30 : i32
     %7 = arith.constant {tag="default_b"} 300 : i32
     scf.yield %6, %7 : i32, i32
   }
   memref.store %0, %m0[] {tag_name = "a"} : memref<i32>
   memref.store %1, %m0[] {tag_name = "b"} : memref<i32>
   return
 }
+
+// -----
+
+// CHECK-LABEL: llvm.func @decl(i64)
+// CHECK-LABEL: llvm.func @func(%arg0: i64) {
+// CHECK-NEXT:  llvm.call @decl(%arg0) : (i64) -> ()
+// CHECK-NEXT:  llvm.return
+
+llvm.func @decl(i64)
+
+llvm.func @func(%lb : i64) -> () {
+  llvm.call @decl(%lb) : (i64) -> ()
+  llvm.return
+}