Check users of instrinsics instead of traversing entire function.

[anna]

Summary

Updated LowerGuardIntrinsic and LowerWidenableCondition to check for
users of the respective intrinsic, instead of checking for guards and
widenable conditions by traversing the entire function.

This is an NFC. Should save some compile time.

[anna]

already landed. closing as done.

llvm/lib/Transforms/Scalar/LowerGuardIntrinsic.cpp

static bool lowerGuardIntrinsic(Function &F) {
   // Check if we can cheaply rule out the possibility of not having any work to
   // do.
   auto *GuardDecl = F.getParent()->getFunction(
       Intrinsic::getName(Intrinsic::experimental_guard));
   if (!GuardDecl || GuardDecl->use_empty())
     return false;
 
   SmallVector<CallInst *, 8> ToLower;
-  for (auto &I : instructions(F))
-    if (isGuard(&I))
-      ToLower.push_back(cast<CallInst>(&I));
+  // Traverse through the users of GuardDecl.
+  // This is presumably cheaper than traversing all instructions in the
+  // function.
+  for (auto *U : GuardDecl->users())
+    if (auto *CI = dyn_cast<CallInst>(U))
+      if (CI->getFunction() == &F)
+        ToLower.push_back(CI);
 
   if (ToLower.empty())
     return false;
 
   auto *DeoptIntrinsic = Intrinsic::getDeclaration(
       F.getParent(), Intrinsic::experimental_deoptimize, {F.getReturnType()});
   DeoptIntrinsic->setCallingConv(GuardDecl->getCallingConv());
 

llvm/lib/Transforms/Scalar/LowerWidenableCondition.cpp

static bool lowerWidenableCondition(Function &F) {
   // do.
   auto *WCDecl = F.getParent()->getFunction(
       Intrinsic::getName(Intrinsic::experimental_widenable_condition));
   if (!WCDecl || WCDecl->use_empty())
     return false;
 
   using namespace llvm::PatternMatch;
   SmallVector<CallInst *, 8> ToLower;
-  for (auto &I : instructions(F))
-    if (match(&I, m_Intrinsic<Intrinsic::experimental_widenable_condition>()))
-      ToLower.push_back(cast<CallInst>(&I));
+  // Traverse through the users of WCDecl.
+  // This is presumably cheaper than traversing all instructions in the
+  // function.
+  for (auto *U : WCDecl->users())
+    if (auto *CI = dyn_cast<CallInst>(U))
+      if (CI->getFunction() == &F)
+        ToLower.push_back(CI);
 
   if (ToLower.empty())
     return false;
 
   for (auto *CI : ToLower) {
     CI->replaceAllUsesWith(ConstantInt::getTrue(CI->getContext()));
     CI->eraseFromParent();
   }