[Support] Do not use llvm::size in getLoopPreheader (#94540)

`BlockT *LoopBase<BlockT, LoopT>::getLoopPreheader()` was changed in
7243607 to use `llvm::size` rather than
the checking that `child_begin() + 1 == child_end()`. `llvm::size`
requires that `std::distance` be O(1) and hence that clients support
random access. Use `llvm::hasSingleElement` instead.

Author: bnbarham

llvm/include/llvm/Support/GenericLoopInfoImpl.h

@@ -208,7 +208,7 @@ BlockT *LoopBase<BlockT, LoopT>::getLoopPreheader() const {
     return nullptr;
 
   // Make sure there is only one exit out of the preheader.
-  if (llvm::size(llvm::children<BlockT *>(Out)) != 1)
+  if (!llvm::hasSingleElement(llvm::children<BlockT *>(Out)))
     return nullptr; // Multiple exits from the block, must not be a preheader.
 
   // The predecessor has exactly one successor, so it is a preheader.