[Support] Consolidate the two implementations of Recycler::clear (NFC) (llvm#165081)

This patch consolidates the two implementations of Recycler::clear
with "if constexpr" for simplicity.

Author: kazutakahirata

llvm/include/llvm/Support/Recycler.h

@@ -19,6 +19,7 @@
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/ErrorHandling.h"
 #include <cassert>
+#include <type_traits>
 
 namespace llvm {
 
@@ -72,19 +73,19 @@ class Recycler {
   /// deleted; calling clear is one way to ensure this.
   template<class AllocatorType>
   void clear(AllocatorType &Allocator) {
-    while (FreeList) {
-      T *t = reinterpret_cast<T *>(pop_val());
-      Allocator.Deallocate(t, Size, Align);
+    if constexpr (std::is_same_v<std::decay_t<AllocatorType>,
+                                 BumpPtrAllocator>) {
+      // For BumpPtrAllocator, Deallocate is a no-op, so just drop the free
+      // list.
+      FreeList = nullptr;
+    } else {
+      while (FreeList) {
+        T *t = reinterpret_cast<T *>(pop_val());
+        Allocator.Deallocate(t, Size, Align);
+      }
     }
   }
 
-  /// Special case for BumpPtrAllocator which has an empty Deallocate()
-  /// function.
-  ///
-  /// There is no need to traverse the free list, pulling all the objects into
-  /// cache.
-  void clear(BumpPtrAllocator &) { FreeList = nullptr; }
-
   template<class SubClass, class AllocatorType>
   SubClass *Allocate(AllocatorType &Allocator) {
     static_assert(alignof(SubClass) <= Align,