Use non-throwing get

Author: mrkajetanp

flang/lib/Semantics/rewrite-parse-tree.cpp

@@ -410,9 +410,11 @@ bool RewriteMutator::Pre(parser::OpenMPLoopConstruct &ompLoop) {
     // call OpenMPSimdOnly on the nested loop block while indicating where
     // the block comes from.
     auto &nest = std::get<std::optional<parser::NestedConstruct>>(ompLoop.t);
-    if (nest.has_value()) {
-      auto &doConstruct = std::get<parser::DoConstruct>(nest.value());
-      auto &innerBlock = std::get<parser::Block>(doConstruct.t);
+    if (!nest.has_value()) {
+      return true;
+    }
+    if (auto *doConstruct = std::get_if<parser::DoConstruct>(&*nest)) {
+      auto &innerBlock = std::get<parser::Block>(doConstruct->t);
       OpenMPSimdOnly(innerBlock, /*isNonSimdLoopBody=*/true);
     }
   }