[Support] Use "if constexpr" in shouldReverseIterate

This patch simplifies shouldReverseIterate with "if constexpr" while
switching to "constexpr bool", allowing compile-time evaluation at
call sites.

Author: kazutakahirata

llvm/include/llvm/Support/ReverseIteration.h

@@ -6,13 +6,11 @@
 
 namespace llvm {
 
-template<class T = void *>
-bool shouldReverseIterate() {
-#if LLVM_ENABLE_REVERSE_ITERATION
-  return detail::IsPointerLike<T>::value;
-#else
-  return false;
-#endif
+template <class T = void *> constexpr bool shouldReverseIterate() {
+  if constexpr (LLVM_ENABLE_REVERSE_ITERATION)
+    return detail::IsPointerLike<T>::value;
+  else
+    return false;
 }
 
 } // namespace llvm