[flang] Avoid unnecessary looping for constants (#156403)

Going through and doing `convertToAttribute` for all elements, if they
are the same can be costly. If the elements are the same, we can just
call `convertToAttribute` once.

This does give us a significant speed-up:
```console
$ hyperfine --warmup 1 --runs 5 ./slow.sh ./fast.sh
Benchmark 1: ./slow.sh
  Time (mean ± σ):      1.606 s ±  0.014 s    [User: 1.393 s, System: 0.087 s]
  Range (min … max):    1.591 s …  1.628 s    5 runs

Benchmark 2: ./fast.sh
  Time (mean ± σ):     452.9 ms ±   7.6 ms    [User: 249.9 ms, System: 83.3 ms]
  Range (min … max):   443.9 ms … 461.7 ms    5 runs

Summary
  ./fast.sh ran
    3.55 ± 0.07 times faster than ./slow.sh
```

Fixes #125444

Author: Saldivarcher

flang/lib/Lower/ConvertConstant.cpp

@@ -145,14 +145,34 @@ class DenseGlobalBuilder {
       fir::FirOpBuilder &builder,
       const Fortran::evaluate::Constant<Fortran::evaluate::Type<TC, KIND>>
           &constant) {
+    using Element =
+        Fortran::evaluate::Scalar<Fortran::evaluate::Type<TC, KIND>>;
+
     static_assert(TC != Fortran::common::TypeCategory::Character,
                   "must be numerical or logical");
     auto attrTc = TC == Fortran::common::TypeCategory::Logical
                       ? Fortran::common::TypeCategory::Integer
                       : TC;
     attributeElementType =
         Fortran::lower::getFIRType(builder.getContext(), attrTc, KIND, {});
-    for (auto element : constant.values())
+
+    const std::vector<Element> &values = constant.values();
+    auto sameElements = [&]() -> bool {
+      if (values.empty())
+        return false;
+
+      return std::all_of(values.begin(), values.end(),
+                         [&](const auto &v) { return v == values.front(); });
+    };
+
+    if (sameElements()) {
+      auto attr = convertToAttribute<TC, KIND>(builder, values.front(),
+                                               attributeElementType);
+      attributes.assign(values.size(), attr);
+      return;
+    }
+
+    for (auto element : values)
       attributes.push_back(
           convertToAttribute<TC, KIND>(builder, element, attributeElementType));
   }