[NFC][SYCL][Reduction] Use different workaround than #7484 (#7500)

This reverts #7484 and uses a less intrusive workaround for the gcc bug.

sycl/include/sycl/reduction.hpp

@@ -2314,15 +2314,17 @@ void reduction_parallel_for(handler &CGH, range<Dims> Range,
 
   // Before running the kernels, check that device has enough local memory
   // to hold local arrays required for the tree-reduction algorithm.
-  size_t OneElemSize;
-  if constexpr (NumArgs == 2) {
-    using Reduction = std::tuple_element_t<0, decltype(ReduTuple)>;
-    constexpr bool IsTreeReduction =
-        !Reduction::has_fast_reduce && !Reduction::has_fast_atomics;
-    OneElemSize = IsTreeReduction ? sizeof(typename Reduction::result_type) : 0;
-  } else {
-    OneElemSize = reduGetMemPerWorkItem(ReduTuple, ReduIndices);
-  }
+  size_t OneElemSize = [&]() {
+    // Can't use outlined NumArgs due to a bug in gcc 8.4.
+    if constexpr (sizeof...(RestT) == 2) {
+      using Reduction = std::tuple_element_t<0, decltype(ReduTuple)>;
+      constexpr bool IsTreeReduction =
+          !Reduction::has_fast_reduce && !Reduction::has_fast_atomics;
+      return IsTreeReduction ? sizeof(typename Reduction::result_type) : 0;
+    } else {
+      return reduGetMemPerWorkItem(ReduTuple, ReduIndices);
+    }
+  }();
 
   uint32_t NumConcurrentWorkGroups =
 #ifdef __SYCL_REDUCTION_NUM_CONCURRENT_WORKGROUPS