[NFC][Coroutines] Build DominatorTree only once before collecting frame allocas (PR58650)

Assuming that collecting frame allocas doesn't modify CFG we can safely
move DominatorTree construction outside loop and avoid expensive computations.

Differential Revision: https://reviews.llvm.org/D140818

Author: yurai007

llvm/lib/Transforms/Coroutines/CoroFrame.cpp

@@ -2609,6 +2609,7 @@ static void sinkLifetimeStartMarkers(Function &F, coro::Shape &Shape,
 static void collectFrameAllocas(Function &F, coro::Shape &Shape,
                                 const SuspendCrossingInfo &Checker,
                                 SmallVectorImpl<AllocaInfo> &Allocas) {
+  const DominatorTree DT(F);
   for (Instruction &I : instructions(F)) {
     auto *AI = dyn_cast<AllocaInst>(&I);
     if (!AI)
@@ -2618,7 +2619,6 @@ static void collectFrameAllocas(Function &F, coro::Shape &Shape,
     if (AI == Shape.SwitchLowering.PromiseAlloca) {
       continue;
     }
-    DominatorTree DT(F);
     // The code that uses lifetime.start intrinsic does not work for functions
     // with loops without exit. Disable it on ABIs we know to generate such
     // code.