ARROW-8130: [C++][Gandiva] fix dex visitor to handle interval type

Closes #6629 from pprudhvi/intervalnull

Authored-by: Prudhvi Porandla <prudhvi.porandla@icloud.com>
Signed-off-by: Wes McKinney <wesm+git@apache.org>

cpp/src/gandiva/llvm_generator.cc

@@ -1,4 +1,4 @@
-// Licensed to the Apache Software Foundation (ASF) under one
+// Licensed to the Apache Software Foundation (ASF) under one
 // or more contributor license agreements.  See the NOTICE file
 // distributed with this work for additional information
 // regarding copyright ownership.  The ASF licenses this file
@@ -708,6 +708,17 @@ void LLVMGenerator::Visitor::Visit(const LiteralDex& dex) {
       value = types->i64_constant(arrow::util::get<int64_t>(dex.holder()));
       break;
 
+    case arrow::Type::INTERVAL: {
+      std::shared_ptr<arrow::IntervalType> interval_type =
+          arrow::internal::checked_pointer_cast<arrow::IntervalType>(dex.type());
+      if (interval_type->interval_type() == arrow::IntervalType::type::MONTHS) {
+        value = types->i32_constant(arrow::util::get<int32_t>(dex.holder()));
+      } else {
+        value = types->i64_constant(arrow::util::get<int64_t>(dex.holder()));
+      }
+      break;
+    }
+
     case arrow::Type::DECIMAL: {
       // build code for struct
       auto scalar = arrow::util::get<DecimalScalar128>(dex.holder());