[Coroutines] Only run verifyFunction in debug mode

[lxfind]

Summary

verifyFunction can be really slow on large functions. This can significantly slow down compilation in production.
Given that coroutine passes are fairly stable now, we should only run it in debug mode.

[wenlei]

lgtm, thanks.

llvm/lib/Transforms/Coroutines/CoroSplit.cpp

auto *DestroyAddr = Builder.CreateStructGEP(
       Shape.FrameTy, Shape.FramePtr, coro::Shape::SwitchFieldIndex::Destroy,
       "destroy.addr");
   Builder.CreateStore(DestroyOrCleanupFn, DestroyAddr);
 }
 
 static void postSplitCleanup(Function &F) {
   removeUnreachableBlocks(F);
 
+#ifndef NDEBUG
   // For now, we do a mandatory verification step because we don't
   // entirely trust this pass.  Note that we don't want to add a verifier
   // pass to FPM below because it will also verify all the global data.
   if (verifyFunction(F, &errs()))
     report_fatal_error("Broken function");
+#endif
 }
 
 // Assuming we arrived at the block NewBlock from Prev instruction, store
 // PHI's incoming values in the ResolvedValues map.
 static void
 scanPHIsAndUpdateValueMap(Instruction *Prev, BasicBlock *NewBlock,
                           DenseMap<Value *, Value *> &ResolvedValues) {
   auto *PrevBB = Prev->getParent();