[NFC][Coroutines] Remove redundant checks for replacing PrepareFns (#98392)

If `Coroutines.empty()` the following loop is going to be skipped
entirely and same goes for `PrepareFns.empty()`. These two conditions
here aren't useful and adds to complexity.

Author: yuxuanchen1997

llvm/lib/Transforms/Coroutines/CoroSplit.cpp

@@ -2108,12 +2108,6 @@ PreservedAnalyses CoroSplitPass::run(LazyCallGraph::SCC &C,
   if (Coroutines.empty() && PrepareFns.empty())
     return PreservedAnalyses::all();
 
-  if (Coroutines.empty()) {
-    for (auto *PrepareFn : PrepareFns) {
-      replaceAllPrepares(PrepareFn, CG, C);
-    }
-  }
-
   // Split all the coroutines.
   for (LazyCallGraph::Node *N : Coroutines) {
     Function &F = N->getFunction();
@@ -2143,11 +2137,9 @@ PreservedAnalyses CoroSplitPass::run(LazyCallGraph::SCC &C,
     }
   }
 
-  if (!PrepareFns.empty()) {
     for (auto *PrepareFn : PrepareFns) {
       replaceAllPrepares(PrepareFn, CG, C);
     }
-  }
 
   return PreservedAnalyses::none();
 }