[ADT] Use "if constexpr" with shouldReverseIterate (#161477)

This patch uses "if constexpr" whenever we call shouldReverseIterate
in "if" conditions.  Note that shouldReverseIterate is a constexpr
function.

Author: kazutakahirata

llvm/include/llvm/ADT/SmallPtrSet.h

@@ -476,18 +476,20 @@ template <typename PtrType> class SmallPtrSetImpl : public SmallPtrSetImplBase {
   }
 
   [[nodiscard]] iterator begin() const {
-    if (shouldReverseIterate())
+    if constexpr (shouldReverseIterate())
       return makeIterator(EndPointer() - 1);
-    return makeIterator(CurArray);
+    else
+      return makeIterator(CurArray);
   }
   [[nodiscard]] iterator end() const { return makeIterator(EndPointer()); }
 
 private:
   /// Create an iterator that dereferences to same place as the given pointer.
   iterator makeIterator(const void *const *P) const {
-    if (shouldReverseIterate())
+    if constexpr (shouldReverseIterate())
       return iterator(P == EndPointer() ? CurArray : P + 1, CurArray, *this);
-    return iterator(P, EndPointer(), *this);
+    else
+      return iterator(P, EndPointer(), *this);
   }
 };
 

llvm/lib/Support/StringMap.cpp

@@ -83,7 +83,7 @@ unsigned StringMapImpl::LookupBucketFor(StringRef Name,
   // Hash table unallocated so far?
   if (NumBuckets == 0)
     init(16);
-  if (shouldReverseIterate())
+  if constexpr (shouldReverseIterate())
     FullHashValue = ~FullHashValue;
   unsigned BucketNo = FullHashValue & (NumBuckets - 1);
   unsigned *HashTable = getHashTable(TheTable, NumBuckets);
@@ -142,7 +142,7 @@ int StringMapImpl::FindKey(StringRef Key, uint32_t FullHashValue) const {
 #ifdef EXPENSIVE_CHECKS
   assert(FullHashValue == hash(Key));
 #endif
-  if (shouldReverseIterate())
+  if constexpr (shouldReverseIterate())
     FullHashValue = ~FullHashValue;
   unsigned BucketNo = FullHashValue & (NumBuckets - 1);
   unsigned *HashTable = getHashTable(TheTable, NumBuckets);